The difference between being believable and being realistic may not be that different between works of fantasy and novels about “real life.” Books about life as we know it are often not realistic but contain an adrenaline-charged or romanticized version of our world. This kind of fiction includes impossible coincidences and people with characteristics or abilities that may exist in less than one percent of the population. In thrillers and mysteries, the reader is often expected to suspend disbelief just as much as when reading science fiction and fantasy.
However, in fantasy, science fiction, alternate history and other “unreal” genres, the author is expected to create a set of rules for his world. The readers need a story that is believable despite existing in another time or universe. This semblance of believability connects it to our world, so the novel does not read as a digression into nonsense. The key to this is creating limitations.
Assume that you want one of your characters to be able to read or even control the minds of other characters, but you don’t want that character to be too powerful. The key to that is including limitations that are common occurrences. For example, listening to one person is easy, at least for a while. Hearing five or six people chatter at the same time leads to the inability to comprehend them, and maybe a massive headache. Hearing the constant thoughts of that many people would likely drive someone insane. This would require withdrawing from a crowd or shutting down this subliminal communication. Limitation to the very unreal ability to read or control minds could be similar to those for hearing and comprehending people that are speaking.
Limitations are just as applicable when writing believable mysteries and thrillers. Creating events viewed through the lens of reality may slow down the action, but more action does not necessarily lead to more tension. The destination of a trap door or the conclusion to a car chase may be in question the first time this device is used. But after the second time a character faces that same situation, readers might start yawning if they see it again. The character’s inability to know how to respond is what builds tension.
I’ve sometimes heard authors claim that characters “take on a life of their own” and start making the choices that they want. That could be the cue that a character is not interesting because he does not have to struggle enough. The writer is still the creator who decides what the character will and can do. The author still weighs which actions would be best for the story and decides what goes onto the paper or into the computer. Characters, like children, need to face limits to grow.