As I put my characters through their paces, I find they're getting mouthy. "Let's see you do it!" seems to be the rally-cry of choice at the moment. Maybe you've never had an overly opinionated character try and talk you out of inflicting some terrible fate on them, but I'd wager you know exactly what I'm talking about.
There comes a point, if you've drawn your characters realistically, that their voices become distinct. They develop their own personalities (much like children, those personalities aren't always what their progenitor expects and/or plans), and you can't help but care for them and what they've become.
Then it happens...
Also like children, they start to think they can manipulate you into getting their way.
They don't like the way they look, so they decide to dye their hair.
They come out of the closet or undergo complete gender re-assignment without notice. They switch nationalities like you'd change shoes.
They share secrets with each other that one will inevitably blurt out at the most inopportune (but dramatically perfect) moment.
Something's too hard, and they don't want to do it, so they whine and scream and cry and throw a full bodied temper tantrum, refusing to cooperate until you cave. (This is where their brilliant coup falls apart, because unlike children, no one else can hear them. Ha!)
Something's boring and they want to do something else. (This one you should probably listen to. If the characters are bored, the reader will be, too.)
Now you've come to the point where you have to decide if you want to keep going with the story you thought you were writing or do you want to let the characters "live" organically. You've given them the voice they use and the past that they draw from, so maybe you should trust that part of you that created them to know best.
It's strange, but exciting when characters come into their own. And the resulting words can be surprisingly perfect in a way you could never have planned on your own.