Most films and quite a few movies portray a character who changes as a result of the things that happen in the story--this is the "character arc." Many screenwriting books and teachers say this is mandatory and that it should be a big, dramatic change.
Writer/director Paul Thomas Anderson ("There Will be Blood") has a take on the character arc that's a bit different from most:
"I guess what I like in my movies is where you see a character change by maybe two degrees as opposed to the traditional movie change of ninety degrees. I guess that always feels false to me in movies because that doesn't truly happen. Around me, at least in the life I live, I guess I don't see people change ninety or a hundred degrees. I see them change in very small increments. I think it's just a monitor I might have on myself as a writer to not make any false scenes."
It's a good reminder that the story you really want to tell comes first, the "rules" come second.