I have been an avid reader (and movie watcher) my whole life and the number one thing that will shoot me right out of an otherwise otherworldly, out of body story experience is when characters do or say something that in my heart of hearts I know they wouldn't. I HATE THIS. I especially hate this when it, shortly thereafter, becomes clear that the creator of this universe (that has just failed to suspend my disbelief) uses this unbelievable doing or saying of something simply to set up a plot point later in the story.
This makes me cranky.
Given my day job, one thing I think I've gotten pretty astute at when working with a kid is (trying) to wade through all their overt behaviors (no eye contact, dirt looks, snotty tone, that chair they just threw across the room) and look for the underlying motivation. This is an important skill, for me, to develop because if all I'm concentrating on is the fact that this kid is refusing to do any school work, is defiant and confrontational with her teachers and will only draw pictures of burning buildings, I might miss the fact that her dad was just sent to prison--for life. (disclaimer: I don't talk about real kids, or their problems, here--or anywhere for that matter.)
Now this next statement is really only valid when considering what pen in the psychological theory circus your like to roll around in (assuming you don't just think it's full of clowns) but I happen to spend a lot of time with the cognitive behaviorists.
And as such, I happen to think all behavior has meaning.
What does this have to do with my writing and more importantly, yours?
Because our characters are people (and even if your character is a dog, alien or rabid space monkey, he needs to experience human like emotions, wants, and dreams in order to connect to your human audience) and these people need to have real wants and then make believable choices that (they think) will help them achieve that want. I don't think they have to be the right choices, the best choices, or even the smartest choices (and it's probably much more interesting if they aren't) but I believe they have to be choices that make sense to your character's personality.
Now by no means am I claiming that my writing executes this perfectly or even all the time. But it is something, as a reader, I am always trying to watch for in my own writing.