Characters that change our storylines

I have a love/hate relationship with my character's relentless ability to change my storyline. I'll map the whole thing out, write each chapter description so that it fits neatly within my story arc, and then, as I actually write the thing, suddenly my heroine doesn't like the way a character says something, and she won't stand for it. It's like being at war with myself- and I can be a defiant little shit.

There's also something wonderful that happens when your characters have finally developed enough to become their own beings. The story tightens, the interactions are more genuine, and the reality of this other world bleeds into every sentence. It's fulfilling to see their personalities take shape, but it's also uncomfortable, because it becomes a battle to reign in these fictional people just enough to keep the story moving, without stifling their self-expression and killing the flow of the book. 

I guess it comes down to this: writing is weird. I'm down to the last couple chapters of my book- almost at the finish line. I know where my characters are going and I have this vague idea of how they're going to get there. Inevitably, they'll fight back against fate like we all do here in the "real world". I can't wait to see what they do with the ending I have planned.