I am talking about the movies that are made on any book. I have read the whole Harry Potter series, and I have watched all the movies – none of the movies do any justice to the respective book. I agree that the Harry Potter books are voluminous, and trying to stuff all that information in a 150-minute movie is a stiff task. But it has happened many times, that friends who have not read the books, while watching the movie are unable to follow the sequence of events. For all these movies, I read the books first – none of the movies were up to the expectations.
The case was opposite for Jurassic Park, I had watched the movie long before I read the book; and I loved the movie. Now on reading the book I feel cheated. So many things have been completely ignored – the breeding grounds and the nests of Velociraptors, their attempt at migration; and the end was totally different. But I cannot complain here, Michael Crichton himself has written the screenplay. Another movie that disappointed me badly was Timeline, I thought it was one of the better sci-fi books, but the movie was really bad.
One movie that does justification to the book is Godfather. The book is awesome, and so is the movie. No subplots, no small details have been ignored. Both, the book and the movie are works of genius. I have not read Lord of the Rings, so cannot comment on that.
I think the authors would be getting mixed feelings when anyone approaches them for movie rights. On one hand it is a great compliment that your book is deemed worthy of making a movie on, but on the other hand the screenplay may turn out to be totally different from what is their in book.