In this great interview of (and about) Robert Gottlieb, Michael Crichton says:
In my experience of writing, you generally start out with some overall idea that you can see fairly clearly, as if you were standing on a dock and looking at ship on the ocean. At first you can see the entire ship, but then as you begin work you’re in the boiler room and you can’t see the ship anymore. All you can see are the pipes and the grease and the fittings of the boiler room and, you have to assume, the ship’s exterior.
This is from “The Paris Review Interviews, 1.”
And yes, I’m stuck in the boiler room, wondering if this ship will stay afloat. Such it goes…