There are many detailed technical breakdowns of FNaF mechanics available online. What I intend to do is a bit different. I will be analyzing the mechanics from a design perspective, presenting a higher-level model of how they generally work, and some underlying principles they tend to follow.
(This post focuses on FNaF1-UCN.)
Screen Transitions and Stalling
Before I get into the broader descriptions of FNaF's gameplay structure, I want to touch on some more specific consequences of the games' constraints.
FNaF is a game with 3D environments and models, but the game itself does not render these models. Rather, all of the 3D visuals are pre-rendered: Scott sets them up in his modelling software, takes a picture, or records a small animation, and puts that picture/animation file into the game. This is useful, because it means Scott doesn't really need to worry about optimizing his models (To my understanding, they are extremely unoptimized). But this also comes with limitations regarding what kinds of mechanics can be implemented.
You rarely see the animatronics move. This is because implementing animatronic movement requires lots of prerendered animations, which are relatively difficult for Scott to implement under this system. Instead, almost all movement is tied to screen transitions, for example, when the camera flips up or down. That's just to say, the animatronics can (usually) only move while you aren't looking.
This simple fact has a huge effect on the way the mechanics are designed. Sometimes, movements are simply hidden by a bit of camera static, but other situations are more complicated. Some characters, for whatever reason, cannot cause camera static, so you can effectively hold them in place just by looking at them on the cameras. This is called "cam-stalling". Many FNaF mechanics are simple contrivances intended to forcefully create screen transitions, so that the animatronics can move.
Here, I will explore how each game works with this constraint.
FNaF1
Here's a fun fact: Though 20/20/20/20 mode is the most difficult possible night in FNaF1, 20/20/20/0 is, with some luck, trivially easy. It can be beaten simply by doing nothing. This is because Foxy is the only character in FNaF1 who can get into your office and kill you without a screen transition. Without him, there is no challenge. (The reason you can still die here is that Foxy's difficulty level increases slightly throughout the night. I did 6 tests, and I only won once, on my second attempt.)Bonnie, Chica, and Freddy cannot move while you're looking at them, which means they can't enter your office to kill you without a screen transition. The only relevant screen transition in FNaF1 is a camera-flip. If the camera is already up, they can force it down to kill you. But when the camera is down, it wouldn't make sense for them to "force it up". For this reason, the player is completely invulnerable to Bonnie, Chica, and Freddy if they simply do not flip up the camera. This tactic is sometimes called "office-stalling".
Foxy is the exception, because his jumpscare just involves him peaking through of the door. This animation flows naturally from the normal state of the office, so it doesn't require a full screen transition. It's important that at least one character be designed this way, otherwise the player could always win by doing nothing. It's also important that the only way to defend against Foxy (in practice) is to look at the cameras, because this forces the player to create screen transitions for the other animatronics to take advantage of.
You can defend against Foxy without the cameras by closing the door, but there are a couple of factors which prevent this from being a viable strategy. Once Bonnie arrives at the left door, he will disable the door controls. This means that in order to use this "do nothing" strategy against Foxy, the player must close the door before Bonnie first arrives, and leave it closed for essentially the entire night. This, combined with the power penalty when Foxy reaches the door, means that the player will almost always run out of power long before the night ends (Though I was able to beat Night 1 this way).
FNaF2
Same story as FNaF1, pretty much. This time, there's 2 characters who can kill without a transition: Withered Foxy, and the Puppet. Their jumpscares feature them leaping from the dark hall in front of the office, which, like Foxy's FNaF1 jumpscare, naturally transitions from the default state of the office.FNaF3
This game has only one real animatronic, so you would think they'd be able to kill without a screen transition. This is, oddly, not the case.Personally, I think this is a strange decision, and a flaw in the game's design. The harder nights tend to result in awkward staring-contests with Springtrap. Foxy's ability to kill you at any time in the previous games made him feel especially threatening, and in my opinion, Springtrap should have been the same way. The fact that he's too shy to kill you until you look away makes him feel like less of a serious threat, and office-stalling is so jank that it makes me feel like I'm cheating.
To compensate, the game has a handful of strategies for forcing transitions whenever it wants. The ventilation system is the main one. It goes off randomly, and you have to flip up the maintanence panel to reboot it, creating a transition. If you don't reboot it, your vision will fade to black, which also creates a transition, so there's no escaping it. The office is also long enough that looking back and forth is a transition in itself.
FNaF4
FNaF4 is where Scott realized he can force a transition whenever he wants just by making the lights flicker. Almost all of the Nightmares work this way.Plushtrap and Nightmare BB are an interesting case. Their minigame is entirely designed around the concept of giving the player complete control over screen transitions (with the flashlight).
Sister Location
I didn't mention this before, but FNaF1 Freddy can actually kill without a transition. His jumpscare animation slides up from the bottom of the screen. The reason avoiding transitions blocks Freddy is that there is a short time between him entering the office, and actually killing you. He still requires a transition to enter the office. If Scott wanted, he could have let Freddy enter the office and kill you without a screen transition, and it would've looked fine. But he didn't, because it would break realism. The right door is in full view from the office, so if Freddy enters, the player realistically should be able to see him walk through the door.For SL, Scott apparently stopped worrying about this. Most of the SL jumpscares slide in from the bottom (or side) of the screen, not requiring a screen transition. In some cases, this results in small realism errors. For example: Ennard can enter the room through the vent, at which point they will jump up from the bottom of the screen and kill you. But how did Ennard invisibly travel from the vent to the bottom of the screen?
Pizzeria Simulator
The main 4 FFPS jumpscares simply slide in from the bottom. The salvage minigame is a somewhat special case. Sometimes the game use the transition from moving the paper up and down. Other times, their jumpscares flow naturally from their previous pose.UCN
UCN obviously has a lot of characters, using a variety of the previous strategies. In general, the game uses the ventilation system as a fail-safe, to ensure that there is always regular screen transitions regardless of what characters are active.Now, we can get into the bigger-picture stuff.
FNaF 1, 2, and 4:
Everything Is a Music Box
Here's what I think of as the "core structure" of classic FNaF gameplay:
There are a bunch of timers, slowly winding down. If any of them hits zero, you lose. You can wind any of them back up, but only one at a time.
The most obvious examples of this in the first 2 games are the music box, and Foxy. The music box is a literal timer that you wind up. Foxy is a metaphorical one: He gets closer to killing you over time (the timer winds down), and you delay him by checking/flashing him (winding it back up). If you ignore him for too long, the timer runs out, and he kills you.
It's a bit more subtle why the other animatronics can be described this way, so I'll use Bonnie as an example. In FNaF1, you can follow Bonnie on the cameras, to keep track of how close he is at any given moment. But this is not only unnecessary, since all that really matters to the player is whether or not he's at the door, but it also becomes less and less practical as the nights progress and Bonnie becomes more sporadic. Ultimately, you just need to treat Bonnie's movement as a black box, and focus solely on checking the lights as often as possible. This is why Bonnie can be described as a timer. Effectively, there's just an RNG that makes him occasionally show up at the door, and the player needs to be there to catch it when that happens. The longer the player goes without checking the door, the greater the risk of Bonnie killing them. Go too long without checking it, and you die; you forgot to wind up the "Left Door Timer".
(SL Night 4 is a very direct literalization of this "juggling timers" concept, and as a result, is essentially an abstracted, bare-bones encapsulation of FNaF gameplay in general.)
So, in practice, almost every animatronic is just a ticking timer you have to keep wound up. It's important, though, that you can only wind one at a time. If in FNaF2, you could simultaneously wind the music box, flash Foxy, and wear the mask, the game would have no challenge. Instead, it's a juggling act.
(Interestingly, Help Wanted seems to entirely misunderstand this, and as a result its adaptations of FNaF 2 and 3 are trivially easy to complete.)
It's easy to see how a simple optimal strategy would emerge from this design. Just go one by one, in a loop. That's it, that's the optimal strategy. Hence, the experience of the early games tends to get repetitive, especially as the difficulty increases, and all strategies are forced to converge to the optimal (if somewhat boring) one. (Interestingly, one person has told me that they find the monotony relaxing, and use FNaF as a comfort/stim game. I may discuss this more in a future post about the autistic appeal of FNaF.)
This structure alone can still create tension, of course. There's always the risk that you might make a simple mistake, and the knowledge that one slip-up could result in nearly instant death. But it's also obviously missing something, so there are a few strategies the games use to spice it up.
Poorly explained mechanics
The player is usually only given an extremely vague understanding of how to deal with the animatronics. This might sound like bad game design, but it definitely works to resolve the issue of repetitive optimal strategies. The player's lack of information regarding the simple mechanics creates gameplay depth, by forcing you to figure it out as you go. The complexity of the decision-making process increases massively when the player is forced to balance many possible theories for how the animatronics might behave. This can create a sense of panic, or powerlessness, which enhances the horror. See Scruffy's video for an audio-focused analysis of this effect in FNaF1.There are some obvious weaknesses to this approach. Too vague, and the player might completely misunderstand, creating frusteration. And as players progress, their understanding will get better and better, making this irrelevant. Absence of information must be sacrificed for the game to actually be playable.
Distractions
FNaF2 introduces a mechanic where some animatronics can fully enter the office, and the player must wear the mask for a few seconds until they go away. This introduces a reaction challenge, but that quickly becomes irrelevant once you realize you can just habitually flip the mask every time you lower your camera. Instead, the real impact this has on the gameplay is as a distraction, by trapping you in a position of momentary powerlessness. You're forced to sit and wait for a few seconds, knowing that all the while, the music box is unwinding, Foxy is getting angrier, the animatronics are entering the vents, and there's nothing you can do about it. It's a simple idea, but it's extremely effective in FNaF2.Dynamical interactions
This is probably the most obvious (and best) way to add depth to the simple timer system: Just make players react dynamically to the information they're given.The doors in FNaF1 are a timer that you must periodically attend to, but that isn't the whole story. It's not a simple matter of checking the lights to reset it, you also have to react to whatever's there, closing/opening the doors if necessary. FNaF4 is the first game to really heavily take advantage of this, with the breathing mechanic (and audio cues more broadly).
FNaF1 Freddy is an interesting case; he's one of the few characters who doesn't fit the timer description very well. He does still "tick down", slowly approaching your office step by step, but his approach isn't something you can reverse. You can't "wind him back up". Instead, he introduces a sort of phase-shift in the optimal strategy, splitting the gameplay into "pre-Freddy" and "post-Freddy" parts. Players must keep track of his position so that when he arrives, they can permanently change their strategy to account for him. Once he arrives, they must be constantly listening for his laugh (or checking the east hall corner) to see whether he's at the door, to close/open it accordingly. It's entirely dynamical, requiring actual, substantive player adaptation depending on Freddy's position. This makes Freddy's position important to keep track of, which adds a whole new information-tracking layer to the strategy.
I should note, though, that if the player knows about cam-stalling, the optimal strategy is again reduced to an almost entirely static routine (which is all but required for 20/20/20/20).
I haven't played many fangames, but I do know that the Flumpty series in particular does a great job with this aspect. All of the characters have some unique behavioral pattern that the player must be specially attuned to in order to defend against them.
FNaF 3, 4, SL, and UCN:
Quick Reactions
Springtrap
FNaF3 featured only one (non-illusory) animatronic: Springtrap. This presents a design challenge: How can traditional FNaF gameplay be adapted to a single-animatronic scenario? The previous games relied on a "juggling act", with each animatronic effectively representing a timer which must be periodically wound up. This framework does not adapt well for the single-animatronic case. Imagine FNaF2, but the only active animatronic is the puppet. You'd just sit on cam 11 winding the music box the whole time, and there'd be no challenge.Springtrap is, predictably, a timer: He "unwinds" as he gets closer, until he reaches you and you die. You "wind him up" by luring him backwards with audio cues. In the early nights, it's just like the Puppet-only situation I described. You stare at Springtrap until he moves, and then play audio lure to bring him back.
What strategies does the game use to solve this problem? Mainly, lots and lots of distractions (As I explained above). Your abilities are periodically reduced by errors, forcing you to spend a few seconds rebooting them. It's largely the same as FNaF2's distractions, but instead of an animatronic stalling you, it's a loading screen. There's also the Phantoms, who cause errors, or just stall you directly in the Puppet's case.
Note that Springtrap does require a good deal of dynamical interaction, because you have to know where he is in order to lure him away.
Nightmare Fredbear and Ennard
There's one aspect of Springtrap's behavior I haven't mentioned: The vents. There are 6 vents Springtrap can enter, indicated by a sound cue, most of which can take him essentially straight to the office. The player must make sure the appropriate vent is sealed, since only one can be sealed at a time. This is another dynamical system: The player must notice when Springtrap is coming from a particular vent, and react accordingly.This is the concept which would be carried over and refined in FNaF4 and SL, for Nightmare Fredbear and Ennard. It is, as it turns out, is a more elegant way to reformulate FNaF's design style to the single animatronic case. Rather than juggling timers, you're in constant loop of picking up cues and reacting accordingly. Miss or misinterpret a cue, and you die. It's very simple, and it has dynamical interactions as its foundation.
Custom Nights
Going forward, this reaction-based style would actually supercede the original, to a large extent. Nightmare Fredbear and Ennard (past 3 AM or so) are very aggressive, and require the player's full attention. But if you take that core idea, tone down the difficulty, and then make several slight variations for different characters, you get SL Custom Night. Funtime Foxy maintains the classic timer style, but everyone else uses this cue-reaction style instead. When you hear Funtime Freddy's voice lines, or Ballora's music, close the corresponding door. When you see Yenndo, flip the camera. When you see Bonnet, click her nose. And so on.UCN generally continues this style, while also throwing in lots of distractions, a few timers, and a few more unique characters. At this point, though, it becomes useful to make small distinction. With some characters, the information is given to you, and with others, you have to seek it out yourself. For example: When Ennard arrives in the vent, his presence is actively announced via an audio cue, and you have to close the vent door. Freddy, on the other hand, moves silently, and the player must go out of their way to seek out the necessary information to defend against his approach. (Although, since Freddy can be office-stalled, you can also simply close the door on each cam flip.) In my experience, there's a pretty big difference in difficulty between these two styles, as one requires the player to do much more mental task management.
UCN overwhelms you with information to keep track of. In a way, UCN's challenge is the exact opposite of the classic games'. In the classic timer style, the challenge comes from the fact that you can only attend to one thing at a time. In UCN, you're forced to attend to everything at once.
This shift in the source of difficulty comes with a pretty substantial rethinking of priorities relevant to the game's core interactions. UCN (and SL Custom Night, for that matter) streamlines everything. Animatronics make a satisfying thump noise when they leave the doorway, telling you exactly when you can open it. You don't close doors with your mouse, but with your keyboard. You do almost everything with your keyboard, actually, which means there's a screen full of button mappings you have to memorize before you can play.
Though this reaction style of gameplay sounds more interesting than timers (and it is, in many ways), it is noteworthy that it creates a much less active gameplay experience. In the classic games, the player must be constantly in motion, always scrambling to get to the next thing as efficiently as possible. On the contrary, SL Custom Night and UCN involve a lot of sitting still, idly staring at the office walls waiting for the next cue.
I have more to say about UCN, but I'll save it for the big redesign post, coming... eventually.