What do writers mean when they talk about their characters
taking over or "listening to the voices" in their heads?
What I mean is that I feel my characters are so fully fleshed out in my mind that I know them well enough to know how they would think, what they would say, what they would do -- if they were real and not just imaginary people in my head.
And often, they become so fully fleshed out that I reach an impasse where I can't move further along a pre-established plot because I've gotten to "know" the characters so well that I know they wouldn't do the thing I originally planned for them to do. So my options at that point are to force them to do something that goes against their characterization, try to redo their characterization and make them different people (this is the literary equivalent of trying to change a spouse after marriage, and rarely works), or let the plot change based on what I've learned about that character(s). I tend to choose the latter, and I tend to get much stronger stories because of that.