People who have never tried writing fiction probably wouldn't believe a lot of the stuff that writers say about "characters." I don't blame them. For instance, some writers insist that their characters "run away with" the story--meaning that the story winds up some way the writer didn't intend. Sounds kinda spooky, doesn't it?
But I've had a mild sense of this happening in a couple of my stories. Not that the story turned out to be wildly different than I had planned, but the more I thought about the scenes from different characters' points of view, the more realistic (and real) their dialogue became. And when one character began to talk realistically, the other characters had to get in line and do likewise or the scene didn't work.
It all boils down to getting to know your characters better. I'm sorry if that doesn't really explain anything either. After all, these are fictional characters. They don't really breathe, speak, or have beating hearts, so how on earth can you get to know them better? I don't know. But you can.