Getting in character

For all the characters in my world, I have at least a basic biography. I don't mean just a description of their appearance and general mannerisms, family history, etc. Each of them have a personality and relationships that I know. The majority of them have little short stories that I've written to better get understand them. They'll never be published but so much goes into each character. Often they'll evolve into completely different people than I first intended during these short stories. Sometimes minor characters will suddenly become so much more than they were first intended. It's funny how things change after the ball starts rolling...