It’s impossible not to think of it from time to time, but in general I try not to, for several reasons. First, it’s possible no movie will be made. Second, the director will decide what parts of the novel will go into the movie, and he may very well cut your favorite parts and add others. So you might as well just write the scenes or characters you want in the book. And third, to worry about practical considerations when you write a chapter (how can they ever film this?) is a waste of time. The experience of movies is that certain apparently simple scenes prove impossible to shoot, and certain extremely difficult scenes turn out to be not so bad, after all.
So in the end, I try to just write a book and not to think about what may happen to it after publication-not only in terms of a movie, but also in terms of what readers will think, what reviewers will say, and so on. Because all that is beyond my control. It happens after the book is published. All I can do is write a book, so I just focus on that.