I don't know about you, but I hate to see a movie or TV show before I've had a chance to read the book. This is especially true if I am reading a series such as Harry Potter or Sookie Stackhouse or Tom Clancy's Jack Ryan. One of the reasons that I love to read books is because I love to use my imagination to "see" the world that the author has created.
When I read a book, I create an image of the characters and how they speak and act in my mind. When I am reading a book that is part of a series, I carry that image with me throughout the series and make adjustments to the character(s) along the way.
When I see a movie about a book, that influences my image of the character so that the next time I read the book or the next book in the series, I see the characters as the actors portrayed them and not necessarily how I originally pictured them. For example, ever since I saw Tom Clancy's "The Patriot Games", every time I read one of the Jack Ryan novels, I picture Harrison Ford as Jack Ryan and hear his voice in my head for the dialog. Yes, I know that Alec Baldwin played Jack Ryan first, but Harrison Ford's version made more of an impact with me.
When I was reading the Harry Potter books, I couldn't watch the movies because I had a picture in my mind of what Harry and his friends looked like and how they acted and sounded and I didn't want to replace that with the actors in the movie, especially if I didn't like them. I bought "Harry Potter and the Sorcerer's Stone" when I came out, but I didn't watch it. I loaned it to my mom so that she could watch it with my sister one weekend. I didn't see any of the Harry Potter movies until after I read book 7, "Harry Potter and the Deathly Hallows".
I have more on this topic, but this is enough for now.