I plot when I have to, but I'm not one of these people who outlines in great detail, because just knowing an outline doesn't really help me to figure out how to tell that story. I usually know enough to make a start on writing, and then jump all over the shop as scenes occur to me, and then have to link everything up in a way that makes sense. That's the point when I usually start to work on the plot as something separate from the actual writing process, because otherwise I can't make the individual sections join up!

And it's definitely the case that the characters develop the story as they develop, by doing unexpected things, or revealing unexpected secrets, which in turn affects how events unfurl.