You hope for the best, that you'll get a good script and they won't suddenly decide to star John Travolta and Bette Midler and change the ending. Two hundred pages of book is about all a movie can handle, unless you're going at rapid fire pace, like His Girl Friday. But eight hundred novels don't usually do well at all. The thing to remember is the book audience isn't the movie audience, and you might actually pick up a reader or two along the way. I love writing books partly because you have total control.
Movies are group projects.
I don't work well on committees, and even wrote a story about it called Why The World Didn't End Last Tuesday, about the archangel committee attempting to plan Armageddon. Then I let myself get really nervous. Or I'm very excited. It's written as a literary criticism essay. Sounds quite good.