I am not one of those writers who feels her characters are "real" and is subject to their whims. (I do experience that sensation of "this action isn't right for this character," but I don't anthropomorphize it. I have my theories about the source of this, but that's for another blog post.) However, I do have some sense of story and world as independent entities - that they are, to some degree, outside of me.
Mostly, this tends to arise when an editor or reader makes a suggestion for a significant change to the story. I'm human, of course: I'll admit that occasionally, my initial reaction is due to laziness, "But I don't want to!" However, other times I balk and can't even, necessarily, articulate why the alteration doesn't work for me.
But it boils down to certainty, a voice inside me that says: "That's not how the story goes." To me, the stories I've written have an existence of their own. I can't change them any more than I could go out and add Elvis to Mount Rushmore. Perhaps it's a subconscious sense of everything that doesn't reach the page, the words between the words. Perhaps those lines are telling me things that an editor / reader can't see. Perhaps it's something else.
I always feel a bit guilty when I turn down a rewrite request, as if I'm being a diva - that I don't have the status to contradict an editor, particularly when my reasoning is no more concrete than, "I don't think this is right for the story." Sometimes it's as simple as, "That might be a valid interpretation, but it's not the story I want to tell." Other times ... I can't even imagine that as the story's reality.
Sometimes, fiction is almost as much about finding as it is about creating.