The book is on simmer--that's what I call it when I'm mulling over a bunch of stuff, trying to figure out how to write a certain section, or determine what needs to be revealed next. A whole bunch of ideas just bubble away until, all at once, something pops to the top that clarifies my direction.
To change the metaphor: writing is a lot like walking into doors. Some are open when you come down the hall, some open just as you get there, and some you aren't sure are closed until you smack into them. Ouch! Not that one right now. Of course, some are also hidden doors that only open when you accidentally hit the secret button. And secret buttons are usually found only in the process of keeping the butt in the chair and producing verbiage. Then, somewhere in the midst of writing a paragraph, the whole next scene opens up for you. But those doors are rare.