It's a phenomenon that cannot be explained. Writer's experience it at one point, and it's a bewildering turn of events.
What do I speak of?
When the characters take over your book, of course!
It doesn't seem possible, yet not only do I have a first-hand account, but I also have testimony from other writers to back up this strange occurrence. One would think, as the writer, we have perfect control over where we take our plots, and what we do to our characters.
It's a lie.
As our characters change from two-dimensional, become more fleshed out and gain a personality of their own, they start to exert their own demands. Sometimes even minor characters demand a larger role, or some more screen time than you planned. Sometimes, as in a case from a writer I have recently met, the antagonist doesn't want to be the 'bad guy.'
So what does a writer do when the characters start rebelling?
Well, that's simple; start listening to what they want! A story shouldn't be rigid in it's structure. Adaptions, changes, and an open mind are three important keys to approaching the art of noveling. Along with bribing the characters of your novel.
A lot of the times the 'characters' will produce great ideas. I have a nasty habit of having plot-holes pointed out by my main protagonist. We have a love/hate relationship in that department (how dare he point out flaws in my perfection?!).
For the acquaintance I mentioned with the rebellious antagonist, she simply killed him off and replaced him with a 'better' villain (she was teased about that for weeks).
But letting characters take the reins can produce great results. Some days it feels like I'm not in control, and good work comes in those moments. It's a truly unadulterated voice, that's greatly in character, and just what I need.
I think that the writers should let the voices take over in their head, at least on the occasional basis!