Samuel Taylor Coleridge, the poet and author, called drama "that willing suspension of disbelief for the moment, which constitutes poetic faith ..."
Any creative endeavor—and certainly any fictional story—is only successful to the extent that the audience can willingly agree to play along with the fictional events as they watch, hear or read about them. It's part of an unspoken contract: the writer presents the audience with a good story, in exchange for the audience accepting that the events within that story will differ from how things work in the real world (e.g., the characters in the fictional universe will think and behave differently from how an audience member would in a mundane, real-life situation).
Simply put, a fictional story does not have to be "realistic," but it also must not do something which the viewer has not been conditioned to expect. Thus, something which is impossible in Real Life has to be made the norm in your fictional setting in order for viewers to accept it. Viewers are usually willing to go along with creative explanations for fictional concepts like magic or sci-fi gadgetry—which is why they don't criticize your faster-than-light wormhole travel system or wonder how a shrinking potion doesn't violate the laws of matter conservation—so long as those concepts feels plausible within the established confines of that story.
By contrast, if the author violates the very parameters which they have established for their own fictional world (e.g., if a character behaves in a way which does not align with their established personality, background and motivations), the viewer may feel pushed beyond the limits of what they are prepared to accept, and so the work fails in the eyes of that particular audience. This applies even to the more fantastical genres—in Speculative Fiction, a viewer's suspension of disbelief can be broken when a work breaks its own established laws (although that requirement can sometimes be relieved if there is a good enough reason for it), or asks the audience to put up with too many things that come off as contrived. A common way of putting this is "You can ask an audience to believe the impossible, but not the improbable." For example, people will accept that a wizard can use a magic spell to teleport across the world, or that a spaceship has the technology to render itself invisible without also rendering its own sensors blind. But the audience will not accept it if, say, a ferocious carnivore just happens to have a heart attack and die right before it would have killed the main character, or that a hacker guessed his enemy's password on the first try just by typing random letters. Without some prior detail justifying it (or one of the Rules listed below coming into play), an audience is going to cry foul, and their suspended disbelief is now at the forefront of their mind.
To put the above in layman's terms: all stories are contrived by their very nature (since the writer has to move characters from plot point A to plot points B, C, D, and so on), but stories can only retain viewers' suspension of disbelief insofar as the contrivances feel justified in-universe. Stories break viewers' suspension of disbelief when the contrivances start to feel forced onto the story by the writer.
There are two major factors that decide how much the audience is willing to suspend their disbelief. One is the very premise of the work that lets the audience know what to expect from the work. The other is the introduction that sets the tone for the work and establishes the rules on how the world works. In more grounded works, the audience's suspension of disbelief can break much more easily. In the other end, if the work establishes itself to be very wacky and surrealist, then the audience may not even question the unfolding events and even the character's motives.
Of course, different people will have different thresholds for what they're willing to accept in a work, and what may break one person's willing suspension of disbelief may not necessarily have the same effect on another.
Most action movies push this trope almost to the breaking point; for the sake of action, the heroes can do virtually anything, given enough Phlebotinum.
As always, the various Rules override nearly all other considerations. When the audience's disbelief, which was suspended during the show, gets reinstated some time afterward, what you get is Fridge Logic. When the means of "fixing" the Plot Hole just end up making it worse and/or opening up a new one, that's a phenomenon known as the Voodoo Shark.
This trope dates back at least as far as William Shakespeare and Henry V, in which the narrator directly asks the audience to "on your imaginary forces work" in order to accept a few actors running around a stage as kings and princes and the battle of Agincourt. The MST3K Mantra is an exhortation to reinstate your Willing Suspension of Disbelief even if it's been broken, because "it's just a show". Similarly, Bellisario's Maxim calls the audience to reinstate their WSOD by ignoring whatever Plot Holes or other inconsistencies broke it, or that those things really aren't as important as the audience member(s) may think.
Incidentally this is one of the more controversial elements of, believe it or not, Professional Wrestling, and is heavily tied to Kayfabe.
When a true story is so bizarre and/or over-the-top that only its veracity enables this, that's Reality Is Unrealistic.
See also:
- Acceptable Breaks from Reality: Sometimes absolute realism can make a work or story boring, disgusting, downright impossible, or otherwise just doesn't work. Breaking from it in those instances, as long as it's not an Ass Pull and is internally consistent, actually makes your story better.
- The Anthropic Principle: Pretty much every story has some fundamental elements which, unrealistic or improbable as they may be, are vital for the story to function in the first place. So like them or not, you don't really have a choice but to suck it up and accept them if you want to enjoy the story. Sure, it might be unlikely that the Great Detective would just happen to be around to solve the incredibly byzantine murder that someone's committed close by, but if he or she wasn't there you wouldn't be able to enjoy watching him or her solve the mystery in the first place.
- Arbitrary Skepticism: Just because a piece of fiction has an unrealistic or outlandish premise doesn't mean that the viewer has to accept everything (inlcuding a complete lack of internal consistency or an Idiot Plot) in it.
- Artistic License: As above, sometimes the correct depiction of something wouldn't work for the story in some way (e.g. in a Cop Show, having everyone do paperwork and eat and drink coffee 90 percent of the time would likely bore the audience, which is expecting drama and action). Generally doesn't break Willing Suspension of Disbelief unless the following applies:
- The Artistic License is so inaccurate and jarring that it doesn't even make sense even in the embellished version of the field (e.g. a cop in said Cop Show doing an on the spot sniff test for DNA)
- The Artistic License is internally inconsistent (e.g. it's established cops in said Cop Show, despite it being over the top action, are still human beings of ordinary strength... and then, suddenly, everyone is dual-wielding rocket launchers and the cops are on the level of space marines.)
- Ass Pull: The biggest way you can break Willing Suspension Of Disbelief. Some subtropes of it will do it almost 100 percent of the time, specifically the Deus ex Machina and Diabolus ex Machina. Generally, this trope and its subtropes needs to be approached carefully and handled masterfully in any form of media where they appear, to avoid pulling the audience out of their suspension.
- Clap Your Hands If You Believe
- The Coconut Effect: Audiences are used to the more unrealistic version of this thing, so including the real version of it is just going to confuse them. Like it or not, you'd be better off not going the "historically accurate" route, and just sticking with what the audience knows.
- Conceit: There are some ideas in every story that just have to be accepted. For instance, if the story is about space explorers going to Mars to meet with aliens, the conceit is that there is life on Mars.
- Contrived Coincidence: If the coincidence couldn't believably happen in Real Life for some reason, try to figure out why and how it could, OR try to figure out an In-Universe explanation for it happening that makes consistent sense. Otherwise, this is right up next to the Ass Pull as a way to immediately break Willing Suspension Of Disbelief.
- Crowning the Criminal: The idea of villains becoming elected holders of office (especially super criminals) flies in the face of logic, but the audience accepts the idea of them doing so because it could show them genuinely reforming, or forcing the public to accept that it was their fault for putting a bad guy in office in the first place.
- Fourth Wall/No Fourth Wall
- Gambit Roulette
- Gameplay and Story Segregation
- Hair Color Dissonance: The audience is meant to accept that an implausible hair color is a stand-in for a normal one.
- How Unscientific!
- Kayfabe: A form of suspending one's disbelief exclusive to Professional Wrestling. The audience knows that the outcomes of these matches are predetermined, and that the wrestlers in the ring aren't really throwing each other around or smashing each other in the face. However, the audience accepts these facts, and enjoys the show regardless.
- Lampshade Hanging: If the characters also agree that something doesn't make sense, the audience will be reassured.
- Literary Agent Hypothesis
- "Magic A" Is "Magic A": If you want to make the audience believe the impossible, you must make it consistent between iterations of said impossibility. For example, if it's established that a wizard can only summon beings weaker than itself, and then the newbie summons a full-blown Eldritch Abomination, you might need to Hand Wave it or establish before that they can pull that off.
- No Permanence, No Stakes: Another consequence of violating suspension of disbelief is that it can cause viewers to stop caring about anything that happens in a story, because if a writer persistently demonstrates that they have no qualms against making up whatever contrivances they feel like, it can dissuade viewers from even bothering to remain invested in anything. (E.g., if a character dies, why should the audience believe that said death will be permanent? Couldn't the author just make up some excuse to bring them Back from the Dead later?)
- Plausible Deniability
- Postmodernism: Postmodernism is, by its very nature, fourth wall-breaking, since the defining feature of postmodern art is its tendency to question the very nature of the medium through which it is being presented, and comment on the nature of similar artistic works which preceded it. If you're looking to tell an immersive story with a engaging narrative, incorporating postmodern elements might not be the wisest thing to do.
- Reality Is Unrealistic: People expect fiction to follow certain conventions and patterns; Real Life doesn't have any such restrictions.
- Sci-Fi Writers Have No Sense of Scale: In the world of the science fiction genre, things tend to be conveniently huge, or conveniently smaller than what they are in reality for the sake of keeping the suspension of disbelief going. A particularly common subtrope of this is the Asteroid Thicket, a dense collection of extremely close asteroids that are somehow not merging together, despite the fact that at this sort of proximity they'd be well on the way to forming a new planet in real life.
- The Treachery of Images
- Weirdness Magnet: One of the ways to explain strange coincidences is that protagonists just have to deal with weird stuff all the time.
