My characters do that all the time. Usually the story ends up pretty much the way I want it to anyway, but they do all kinds of things I hadn't planned in the meantime. That's why I leave a lot of wiggle-room in my plot outlines, to allow for things like that.
As a matter of fact, in the story I'm currently writing, "Twins", Superman just did something I hadn't planned on at all. Oh well, it will just make something I had planned on later happen in a slightly different way ...
Nan
