I also love casting them in my head and more recently on my blog. There are a number of movies that have outdone the book. This doesn't happen too often.
Bridges of Madison County
I really didn't care for the book. It was excessively flowery and wordy. It was a small book, but it felt longer. The movie cuts through the crap and gets to the heart of the story, a farm wife's fling with a traveling photographer. It doesn't hurt that Meryl Streep was the lead.
I love the added scene where Meryl has a fit and pushes Robert nearly accusing him of almost using her. She is wise enough to know that the romance of those few days wouldn't survive in real life. She chooses to cherish what happened and let him go. Hate the book. Love the movie. Thanks Meryl!
Witches of Eastwick
Maybe it was the casting. But the book didn't have the question that the movie had. Did these women somehow conjure this man? The movie was fun and had great performances. The book was convoluted and the women were not in power. In the movie, there is no doubt. I love the ending when they have relegated Jack to the television.
In the movie, they make the magic real. They bring the aunts up as wacky old women rather than dangerous influences. It gives a mythology that the book lacks. It is more fun. The book is dark and pessimistic.
The list of movies in which the book is better than the movie is too long. I think it is just that it is rare for a movie maker to capture my imagination and make it more than it was. There are lots of pretty good adaptations.
But usually, the book is better. Do you have a movie that you liked better than the book?