Editing documentary

I became interested in the making of movies when I took a video editing class in 7th grade. I knew editing was important, but after watching the documentary last wednesday it has completely changed my view of it. One quote that really got to me is when the director of a movie (forgot which movie) said that after writing and filming the movie he was so excited to get it in the editing room so that he could finally make the movie. I always thought that when the movie was being filmed the director and editor already knew exactly how every scene was going to be edited. Apparently this was completely incorrect.