The quick reason, if you've got an appointment, is math. Now run along. For those of you still with us I want to break down a few of the reasons the majority of book adaptations take a header on the pavement. Caveat Reader: Harry Potter and The Lord of the Rings are two giant exceptions, I know. But you don't remember Little Children do you? Or Everything is Illuminated? Or Shopgirl? See, for every Cider House Rules there are three more book to movie carcasses lying around.
The first reason, and the main one, is sheer mathematics. How does math play into the equation you ask? Well, most books are around 70,000 words and 250 pages. This is the bare minimum to get something published these days, and some books being adapted, like Shantaram, run into the 1000 page range. Hollywood doesn't accept scripts over 100 pages, because each page is a minute of filming. Again, there are exceptions such as The Green Mile, (which runs around 180 minutes) but most studios hate longer movies because it means less showtimes (and thus less money). So on one hand the publisher wants a longer book and on the other the studios want a shorter screenplay. This natural conflict of interest makes for rough sailing down the road during the adaptation process. Much will be left out. The story will suffer.
Voice: Most movies are adapted by someone other than the novelist. They are very different art forms, and to be considered proficient at both is a difficult thing. This is why Steven King has only written one screenplay you've ever heard of (Pet Sematary, 18 years ago) but has authored stories and books that everyone's familiar with (The Shawshank Redemption, The Green Mile, The Shining, Stand by Me). Because of this separation of novel and screenplay the person called upon to transform it is not the original writer. In the case of Little Children the director seemingly didn't like the entire tone of the book, so viola, new story (yet under the same name)! Characters also have to get chopped, and the person deciding what's important and what's not doesn't always know what the original writer's intent was.
Dialogue: I realized this one a few weeks back when I was talking to a novelist who had adapted other novelist's work too. He mentioned that you could get away with dialogue in a book that wouldn't play on the big screen. This was a revelation to me, but it makes perfect sense if you think about it. In a movie an actor has to sell you on the words, and it's easy to roll your eyes at sentiment. On the page the author doesn't have to work as hard, if you're already into the book you're already invested in the characters, and thus less likely to find fault with their words. Additionally, when you read a book it's in your own voice, you make the words work the way they make sense in your head, which of course you're going to have an easier time swallowing.
Visual vs. Text: A novelist can very easily spend a page describing a city or a species of plant. A director has about three seconds per shot on average to show you something, and this doesn't allow for detail. A writer who meanders through a story can be charming, and it's okay to talk about a family tree for pages on end. A director or screenwriter who meanders through a tale is a dead duck. Once an audience yawns the battle is over.
This is why comic books will always be an easier medium for the movies than best selling books. A short and visual story? This is a screenwriter's dream. A 1000 page opus to the human condition? It's just a story waiting to hear those fateful words, "Eh, the book was better."
LaremyLegel.com - I've written novels and screenplays, none of them headed towards a bookstore or theater near you.