In the movie Harry Potter and the Chamber of Secrets, it's clear that a number of the characters suspect that Harry is behind the attacks against the Muggle-born witches and wizards at Hogwarts. What isn't clear is that Harry suspects himself, and that he has cause to, which lends a lot of energy to the book. The lack of this key element makes the movie scene where Harry refuses to tell Dumbledore about the voices he's been hearing irrelevant and silly, whereas in the book, Harry's decision is internally consistent.
In contrast, many of the major changes to the Lord of the Rings movie revolve around the ring, and the power it has to corrupt individuals. In the books, the power of the ring is intensely subtle, not manifesting its physical effects on Frodo until well into the third book. This wouldn't work on film. The ring's threat has to be made external, and everybody, from Aragorn to Gandalf, have to be shown to fear the ring's temptation. This is why Aragorn lets Frodo go, because he knows that, given time, he could well pull a Boromir. This is also why Faramir has to take Frodo to Osgiliath; otherwise, it would stand out that this powerful ring would have no power over him, when it has power over everyone else.
Books are internal, while films are external. Films sometimes have difficulty getting inside the characters' heads, unless they make what's internal to the characters something external. That's the difference between staying true to the letter, versus the spirit of the book.