This is something I realized when I saw Eragon with my little brother. My best friend absolutely loved the book, and I'd read an article about the author years before, so Eragon was high on my list of books that I wanted to read. But then I saw the movie. Due to the length of the book, the film obviously had to cut several things out, but the screenwriters and director, in my opinion, made some bad choices in that department. As a result, I took my little brother to see a movie filled with plot holes, poor characterization, and overly contrived circumstances intended to move the story forward. I was so disappointed that it turned me off to the book.
I think it's great when a book can get made into a movie that inspires fans to read more. Harry Potter did a GREAT job with that, but the biggest advantage Rowling had with those movies is that every change to the story had to be approved by her. As a result, the core of the story remained in tact, and anything unimportant could be cut to preserve the spirit of the books.
Not every author has that option, but it seems like so many writers want their books turned into movies that they're willing to sell the film rights to anyone who's willing to put it on the silver screen. Regardless of how good or bad the movie might be.
I don't know how many people decided not to read a book based on how bad the movie was, but I do know that a great movie will inspire me to pick up anything the author might have written.
What are your thoughts? Do you read books that were made into movies?