What the characters do should be based on who they are, which should be based on what they wrote down. If you write down that you're character has a very strong defensive nature, he defends his friends when insulted by a drunkard in the tavern. It also can mean he opens his mouth to the duke when the duke calls his friend a 'street rat' or whatever. Yes, you can develop a personality while playing, but you won't claim you became "loud-mouthed" or "defensive" out of nowhere. Most people would claim that trait had been around for awhile, and thus, have a background beyond motivation as you strive for.