I think writers of speculative fiction do a tremendously good job at bringing their characters, settings, society, and laws governing nature, to life. Readers of speculative fiction, especially science fiction and fantasy, allow a fair amount of leniency in the believability area…I think that is simply the nature of the reader. For example, let’s take the movie Highlander. Here we have these quasi immortal men dueling with swords until there is only one remaining to win some prize. Believable? not hardly, but it’s speculative fiction and therefore we know it’s not real. Now, as fiction readers, we can say: “Okay, what if there were these quasi immortal guys hacking off each other's heads?” If we accept that as a given, then we can sit back and enjoy the story. This is what the readers of speculative fiction do. If you can’t get past the premise of the story, you’re not going to enjoy speculative fiction.
But, believability has layers within the story. Using my example of Highlander, there is a scene where the police are investigating the decapitated man in the underground garage. For whatever reason, a cop is using a metal detector and finds a piece of the sword lodged in the support column. Um…believable? no, not really. We know the story is fiction and therefore allow some leeway, but you can’t get away with using a metal detector in a building on a column that has most certainly steel rebar for support. As readers, we say “no way”, and the story begins losing credibility.
I personally like to spend a fair amount of time on believability factor of the little things. Where the reader says: “Oh yeah, that makes sense.” When I’m writing a scene, I always try to ask myself if the actions or objects make sense. Why does Jericho, in Jericho Solus, allow blood tests after he’s pulled from the ocean? Oh yeah, we find out later that he believed it would keep the government from watching him, as he knew there was nothing in his blood to cause suspicion that he wasn’t human. I think it’s defending and describing these little details that bolster the overall believability of a story.