Willing Suspense of Disbelief
I've been told by some writers that because fiction isn't real, it doesn't need to be realistic or believable. The first problem comes from the fact that fiction doesn't mean "not real", it means "didn't happen in real life". So, is it too much to ask of a writer to put some sort of reality to their writing and to make it believable?
Mark Twain didn't think so and says this. "The personages of the tale shall confine themselves to possibilities and let miracles alone; or, if they venture into a miracle, the author must so plausibly set it forth as to make it look possible and reasonable."There is something in fiction writing that we call "willing suspense of disbelief" and it is a very important part of writing fiction.
Why does a story need to be believable? Well, if your story is based in reality with no elements of the science fiction or fantasy genre, then having pigs suddenly have wings and flying will turn a reader off. Well, it won't turn all readers off. Some readers don't bother to think about what they read. I guess way to put it is these people don't like thinking at all.
This isn't to say these people are bad people mind you. Some may have a learning disability that makes it hard to think of things in a logical manner. They can read more complex stuff, but remembering things just goes over their heads. Other people haven't been taught how to properly thought so thinking isn't an easy thing for them. Other people want their minds numbed for awhile.
Original fiction is less likely to have willing suspense of disbelief because the writer makes the rules of their universe. They are restricted only by the genre and basic logic that goes with any story. For example, science fiction will always have some grounding in actual known science. Fantasy will have rules governing things so that one characters power doesn't overwhelm another person.
Fanfiction is harder because the world is already created by someone else. So, the way that the theoretical science works or the magic works has already been set down in stone, as are the characters personalities. A writer can work with what ifs, but in the process they have to still work within the frames of willing suspense of disbelief.