• The publisher not only accounts for the number of copies of that fancy movie tie-in edition when gearing up for book sales around the movie release date, but also tries to factor in how many copies of the original edition they'll sell due to the publicity surrounding the film. Yes, both the movie tie-in version with that shiny screen shot cover and the original version get a nice sales push from the movie. (This is more pronounced if the book has already been published in paperback form.)
• Even if the reviews for the movie are uniformly abysmal, there will still be a bump in sales for the book. No one is quite sure why this is, but my theory is that most people bank on the book being better than the movie (or, alternately, they want to see whether the book was just as bad).
• As far as I can tell, there's no real second jump in sales when the DVD of the film is released.
Many of you have asked how the movie tie-in phenomenon is figured out in-house. It depends on a lot of factors, but the most important one is: the film rights for your book have to be optioned and somebody (somebody pretty important) has got to turn your book into a movie, which can take years. And that's just not my department. I'll do some research to see what I can dig up, but the honest truth is that without a novel that will translate well to the screen (which is very different from a generally brilliant, salable novel), you're pretty much out of luck.
And besides, if you're writing books in the hopes of having them made into movies... why not just cut out the middle man and write screenplays?