"If a story ends up just like you imagined it would, you've done something wrong. It should surprise the reader AND the writer."
Susan Orlean made this great point on Twitter the other day. It's what I've always known as a variation of the "blue book" phenomenon. During college exams while answering essay questions in those little blue books an idea would present itself and surprise me. It would be one of my best ideas. Usually it would come a little out of order so I'd have to draw big arrows to the portion I'd written before where the idea should logically sit.
This surprise is also a big barrier to writing. If I've been working on a story for a little while and the surprise hasn't happened I start to worry that it never will arrive. I put out cookies and milk. I promise to be nice. Once it does, it's this enormous gift that gets me through the rest of the process.