Back from various real life runnings-around. Something that lodged in my head along the way was a complaint someone had about a Crichton-esque SF-tinged thriller currently cluttering up airport bookstalls: "This read like it was written to be filmed." I didn't think that was an accident.
Most books aimed to take a place on the best-seller lists significantly increase their chances of being picked up in the first place if they read like a movie. This is not just because people like a fast, entertaining read that requires them not to stop and think too much; it's because publishers love the idea of being a party to something that stands a chance of becoming a multimedia franchise.
Here's the problem I have with this: A book is not a movie. A book is a book, and it should be written like a book. A book can take inspiration from the things movies do, but it should not try to imitate the things only movies can do. Movies are great at hitting us with images, moments, theatrical displays of character. They're also good at generating intimacy and empathy. But they don't lend themselves to stimulating thought, except maybe in the sense that certain movies inspire the kind of discussion amongst fans once reserved for doctoral dissertations.
A book is not a screenplay, either. If you write a book as a prelude to it being filmed, that's fine if that's all you want — in which case, maybe you should just cut to the chase and become a screenwriter. But that means you're depriving yourself of all the things only a book can do. I came to the conclusion a while ago that the reason all the film versions of The Great Gatsby have failed in one way or another is because you cannot film something that takes place in a privileged space between the reader and the narrator. (I'd argue you can't really film The Catcher in the Rye for the same reason.)
If someone says that a particular work is "unfilmable", they are not denigrating it. They are paying it respect it is due. Do yourself, and your readers, a favor; write something unfilmable.