The excellent Arts & Letters Daily links to Yeah, but the Book Is Better, an insightful piece by Thane Rosenbaum, whose novel Second Hand Smoke is being adapted to the screen. A key quote:
With a novel, the author forms an implicit partnership with his audience. He provides the story and its voice, but the reader adds the visuals. The power of a novel's description is often tempered by sketchy details. Much is left out in order to leave something to the imagination. The reader is free to conjure the characters in his own way, to picture how they look, because the mind's eye has a way of assembling an image that is quite different from how a character might appear on screen. In the end, the novelist surrenders his book to his readers. Thereafter it becomes theirs, and his proprietary interest ceases.
But read the whole thing. (My own view, for what it's worth, is that short stories and novellas adapt better, because the screen writer has room to flesh things out rather than cut, cut , cut. The Man Who Would Be King and Angels & Insects are two good examples. With a novel what makes it to the screen is a Cliffs Notes version.)