A: There is no right answer to this question. That’s because different people read books and see movies differently. The outcomes are rarely the same. So the real answer is, it's up to you. But here is my "professional" take on it:
I tried reading a book before seeing the movie and I was disappointed in the movie. Because the book can cover more and get more deeply involved in point of view and character development, not to mention the movie not always matching the book, I found myself criticizing the movie more and catching errors or discovering missing pieces of the story instead of simply enjoying the movie.
My best experience has come from seeing the movie and then reading the book. In that way, I get to enjoy the movie for its own sake, and then I read the book to either get more background or to see how the two were different. I’ve never felt disappointed in either version this way.
I bought the book The Constant Gardener in anticipation of seeing the movie first and then reading the book. But the movie didn’t stay in theaters very long and I didn’t get a chance to see it. Yet, I didn’t want to read the book first, so I waited for the movie to come out on DVD. It took much longer than I thought it would and the book languished on the shelf. The movie recently arrived at my favorite video store and I rented it – now I look forward to reading the book. The problem with my seeing the movie first is that I have had to wait to enjoy this great story. So it cuts both ways.