I saw this post over at The Incurable Disease of Writing today. It's called "I have no way, and therefore want no eyes; I stumbled when I saw." The post itself is a reaction to an interview in The Writer, and I have to say that I could not agree more with the original interviewee or Missy from The Incurable Disease. I don't have much to add to her post; you should just read it for yourself.
I will say, though, that I treat anything as an absolute. I do plenty of research and character design and worldbuilding and so on -- both before I ever start writing and consciously as I go -- but my initial outline is never even close to how the book comes out in the end. The characters always take me in unexpected new directions, and new ideas occur to me all throughout the writing process. And yes, I find it impossible to move on if my previously-written chapters have not been edited to some fair degree. Sometimes there's nothing to do but just plunge ahead with nothing but a long-term plan, good worldbuilding, and strong characters -- in such situations, the characters themselves will almost invariably show you what their story will be. And, you know what? I think that's how important parts of the most unique stories come about.