There seem to be two camps of authors out there: those who say their characters are basically real in their head, with the ability to dictate the story and tell the author where to go, and those who scoff at the first camp as utterly ridiculous, since that character is nothing more than a figment of the author's imagination, and therefore has no opinion or idea that is not, in fact, the author's own idea. I say both camps are actually one and the same, just with different perspectives.
I think it is a bit silly to believe that the imaginary people that I create somehow occupy a part of my brain separate to that which is me, and that they can, from their thrones in my head, dictate where the story should go and what they should say. At the same time, a character, once created, is never all me. She is never again just my imagination. She becomes the whole of her parts, and her experiences and dreams dictate her actions, just as mine do in my own life.
So when authors of the first camp say their characters took the story in a different direction than they planned, or that they took over, I believe this is nothing more than the summation of the character's personality traits making a decision that feels right to the character created. It is the author's feel for the rhythm of the characters and the story that makes the moment feel right or wrong. The author may originally have planned a different decision for their character, but by the time that point in the story was reached, it no longer feels true to the character.
For example, in writing Sasha, I preplotted many of the major scenes with quick notes on 3x5 cards. If I had more of the scene envisioned than would fit on the card, I would type it up in a Word document to remind me of my vision weeks later when I reached the scene. Sometimes these notes were nothing more than an extra sentence or two; sometimes these notes were whole pages of dialog and descriptions.
I recently arrived at a scene I'd entitled "Confrontation" because my main characters were to have a very heated argument about their mutual betrayals. I'd written out half a page of their verbal volleys back when I was storyboarding, but when I read back through them now, nearly five weeks later, it no longer rang true for the characters. The scene needed something quieter.
Were I an author of the first camp, I'd say that Eva refused to be pigeonholed into that conversation and had a mind of her own for what needed to be said to the male lead. Were I an author of the second camp, I might claim that this change of plans was nothing more than me changing my mind. Instead, I feel that my decision to go with a different mood for the scene was one of those magical moments when the characters felt like real people, and their decisions—down to when they picked a fight to when they chose to deflect one—felt as clear and real as my own emotions, and I accepted it for the blessing it is, hoping that when I do the first edit, it reads as true as it feels now.