This is precisely what I'm finding with my novel. I made pretty extensive notes before I started writing, in which I outlined everything from the social structure of the society to the characters to the setting. I knew where it was going to go and who was going to be in it--or so I thought. So what has happened? Well, a totally unplanned for, undreamed of character has not only manifested and asserted her intent to stay, she has also announced herself as a pretty major character. As usual, I find that I don't own my material, it owns me. It humors me by letting me think that what I'm writing is my idea but, really, it lacks fingers, so it is reluctantly allowing me to type it out.
A cookie to the first person who correctly guesses this character's name!