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I enjoyed Jurassic Park: The Movie more than Jurassic Park: The Book. I saw the movie first, and I'm glad I did, because it made the book a million tims easier to understand, considering I was pretty young at the time. Some things NEED to be visual. I'm faced with an upcoming dillema: choosing between the LOTR books, and the LOTR movies. If they're as good as I hope they will be, I just may enjoy them more...BUT, If I do enjoy them as much as I think I will, it will surely be because I have already ready most of the series. So, in short, It's impossible for me to tell which is more enjoyable.
It's an odd issue, but I don't think there is any one area where books are completely superior to movies. Just as you said, the casting in your mind (how the character looks and acts, and says their "lines") is always what you consider to be good, but at the same time, the mind cannot visualize things as well by simply imagining them.
Basically, a poorly acted movie loses out to even a half decent book, but a well acted movie can blow a book out of the water, because you have most the benefits of the personal, high-quality cast in your head, but you get to see them vividly.