Which we can change if we like, so it doesn't matter if they do(though you brought to mind that scene in Gorey's "the Unstrung Harp" when he meets his character at the top of the stairs and sees details he hadn't devised). Thing is, though, it's all coming out of you, whether consciously or not, so characters don't really do that, I think: it's an illusion based on our(probably necessary) ignorance of how our creative processes actually work. It's more like you end up coming to different conclusions while "in process" than you do in abstract planning: things change when you're doing it "hands-on." I mean, there have been many times a story took an odd turn just because when it came time to write a scene, I found what I'd planned bored me at the last minute. The work changes as your mind does.