Do you ever find that your characters start to boss you around? Have you ever started writing a story with specific intentions, but your characters refuse to cooperate?

I have often found that characters I write take on a life of their own. They do things I don't want them to do, and take on character traits I never intended for them. Quite often a story will come out completely different to how I originally planned it, simply because one or more of the characters had a will of their own.

So, does this happen to anyone else, or am I just crazy?