Suspension of disbelief is an aesthetic theory intended to
characterize people's relationships to art. It refers to the alleged willingness
of a reader or viewer to accept as true the premises of a work of fiction, even if
they are fantastic, impossible, or contradictory.
Now, it's really been ever since Crossroad that I've been mulling this over, but it only really came to a head recently. It has to do with what people will actually accept in their suspension of disbelief, and what they won't. In the end, I came to the conclusion that willingness to suspend disbelief for the plot varies in direct relation to how much like our world the story world is meant to be.
Accepting the essentially impossible:
To borrow the example a friend of mine used last night, take Superman. While it's set in a world that resembles ours, it's essentially a work of fantasy. Certainly, you could make the argument that science is slowly working towards giving men Superman-type abilities, starting with such things as bulletproof vests to stop bullets with one's chest, but we're expected to accept that he can do these things with his natural body. Still, even though no man is actually able to perform under his own power the feats that Superman can, most people can accept the premise of a man who is "faster than a speeding bullet, more powerful than a locomotive, able to leap tall buildings in a single bound". I would submit that things like this are so fantastic that they cannot disturb our concept of the real world, and thus we can accept them as a premise for fiction.
The same concept would apply to any number of other things: Faster-than-light space travel, aliens with the power to disassemble and properly reassemble the human body practically from scratch, wizards throwing fire or ice or calling down lighting from a clear sky, mahou shoujo or giant robots and their 14-year-old pilots attempting to stave off the annihilation of the earth, and so forth.
Accepting the possible, but outlandish:
Most of the examples I can really come up with here are from science fiction. Perhaps I'm just not reading the right fantasy, but there doesn't seem to be a real equivalent to "hard" science fiction in fantasy. In any event, what the reader is being asked to accept here is something outside of their own experience that either is possible, or can be reasonably said to be possible in the future.
For explanatory purposes, take the idea of humans colonizing outer space. While humanity certainly isn't at that point yet technologically, it isn't impossible to consider ways in which it would be possible. In fact, this case may not be so much a suspension of disbelief as it is a willingness to believe in the creativity and ingenuity of mankind. Still, it falls under the general umbrella.
Likewise, consider the case of an outlandish but possible character: Sherlock Holmes. The reader is asked to believe that Holmes' mental prowess and knowledge far exceeds the norm when it comes to his chosen fields. Here, the problem is not so much believing in the idea that a person could be near or at the top of their field of expertise as it is believing that such a wealth of knowledge and near-perfection could come together in one man.
Accepting the Normal:
This should be fairly obvious. Most people should be able to accept what appear to be normal occurrences, even if they fall outside the realm of an individual's experience. This extends both to plot points (e.g.: a character becomes sick, or is out of town on business) and characterizations (e.g.: the neighbor across the street who gets a beer or two too many in him on a Saturday afternoon, a needy girlfriend who is constantly calling). Even someone too young to be in the workforce, or who has never had a friend or acquaintence like the ones listed can accept and rationalize their existence without any difficulty.
What Qualifies as Unacceptable?
If we're capable of rationalizing both the normal, and the extremely abnormal, why is a term such as suspension of disbelief even necessary? Obviously, there must be points beyond which this concept will not operate. These points may well be different based upon the reader in question, but I would submit that they fall into two primary categories.
Things Which Are Supposed to be Normal, but Are Not:
Going back to Crossroad again, because it is the best example of this in recent memory. To explain it in general terms, it works like this: The world is supposed to be our world. Natural, rational, easy to accept. In fact, there is even a saying, that "life is stranger than fiction", which should cover such things. However, what the author has asked the readers to accept is a cut above. "How these characters got together, while unusual, is not impossible." It may not be impossible, but it is so exceptionally unlikely that the situation in question could happen more than once, perhaps twice, that accepting that it happened four times with the same person involved belies the idea that the world is like ours.
This is more normal-seeming on its face, because it only involves people, but it doesn't really come off any differently than if an author were to write a book set now, in our world, with our current level of science, asking us to buy the fact that humans had developed faster-than-light travel.
Excessive Contrivance to Make the (Nearly) Impossible Possible:
This is probably the more common way that suspension of disbelief is abused or defeated. Most of the time, it boils down to an excessively contrived luck. Put in fantasy terms, our hero just happens to be exactly where he's needed every single time in order to prevent the great evil from dominating the world. Even this we can possibly accept for the sake of the story, as long as it isn't made blatantly obvious.
Brought closer to reality, however, the problem becomes obvious. Instead of the dashing hero in the previous example, consider instead the case of a plumber: On his way home from work, he survives a head-on collision with an 18-wheeler, then, in the hospital, he learns that his wife's cancer has gone into remission, and a couple of weeks later, he's won the state lottery. None of these are excessive in and of themselves, but as they pile up, it becomes more and more difficult to accept that the plumber is a normal person in our world.