An article in the New Yorker (Dec. 5, 2011) quotes writer-director Michel Hazanavicius, whose movie The Artist is a hot prospect for an Oscar for Best Picture. This is what he says about the first part of a movie:
“You have fifteen minutes to tell the audience, ‘These are the rules.’ Jurassic Park teaches us to expect a T. rex, but if a T. rex comes thirty minutes into When Harry Met Sally you won’t believe it.”
I think this applies equally to novels—you have about fifteen to twenty pages to orient people to the world you are creating. If there’s something about it that is different from what we might expect based on those pages, you should at least foreshadow it.
This doesn’t mean you shouldn’t have surprises—quite the contrary. But they have to be plausible in the context you have established.
This includes the characters you have created. If they act out of character, we expect to find out what causes the change. In many action pictures, it’s a threat to someone they love. In comedies it might be getting drunk. In horror pictures it could be a demon that takes up residence inside them. By the way, the argument that many people are inconsistent, while true, isn’t a good defense—we expect people in films to act with more consistency than in life.
This is a good first thing to check when you’re writing your second draft—if something occurred to you as you were writing the later sections that significantly changes the nature of the story or the character go back and adjust.
(There are many more writing tips in the book, "Your Writing Coach," available now from Amazon as a paperback and in the Kindle store.)