The characters, I suppose, start the book for me. They define the main conflict, then I am off. I usually rewrite a section from four to eight or ten times before it is doing what I want, occasionally more. I used to throw out two scenes for every one I used, but I've gotten more efficient. When I start down a wrong path, I tend to recognize it sooner and stop or redirect.
Research also feeds a book.
When I'm stuck, taking a day off to read inevitably sends me back into the story again. Oh, by far the most fun I ever had was traveling to Nice, where I stayed with a dear friend who was working on a film there as a production accountant. Oo la la, the French Mediterranean is finer than fine. I was simply in heaven. And my friend was championship-supportive.