When I first became interested in LotR, one of the first questions I asked was, “Did anyone make a movie of this?” When I discovered that not only had there been made movies of it, but that those movies were three hours long and very good, I was extremely excited.
Considering the movies are three hours long, they put in a lot of details and made it very close to the book. However, there are some differences.
One of the bigger differences I noticed (I think this is only in the Extended Version, though) is the fate of Saruman. In the book, he and Wormtongue are reduced to beggars on the road; in the movie,he and Wormtongue are both killed at Orthanc. I prefer the book because I think it's a good allegory for happens when you misuse gifts God has given you, whereas in the movie Saruman is just another bad guy to be killed.
Another big difference is the romance between Aragorn and Arwen. In the movie, she and Aragorn seem to be closer, but she comes nearer to leaving him behind. In the book, you don't get to see too much of the interaction between them, but she's more certain about staying with him instead of going with the rest of the elves. I think I actually like the movie's portrayal best because you can see more of their feelings towards each other.
The third difference is in the Ents. The “slow and steady” aspect of their character is mostly cut out of the movie. Now, of course you can't spend an hour of the movie on the Ents' meeting, but I think they could have been portrayed better than they were. Also, I would have liked it if they'd included the Entwives. So (obviously) I prefer the book.
The last difference, and the only one that can be really annoying sometimes, is Faramir. You may have gathered that both Eowyn and I are humongous fans of FARAMIR. (In fact, we've even made a pact to- oops. Penderwick Family Honor. :P) In the movie, he's basically just a wiser version ofBoromir, and a lot of his story is cut out. Not to mention the romance between him and Eowyn, which is shown in just a few clips and not given the development of Aragorn and Arwen. (Unless you're watching the Extended Version- I believe there are a lot more scenes between them in the EV.) Predictably, I much prefer the book here too.
Of course, there are many other differences, large and small. However, this is overall an extremely faithful adaptation of LotR and I love it very much.