I don’t believe any of them have actually read it, but rather watched the WETA production on film.
In the books Frodo was a wealthy middle-aged hobbit. Not some boy hobbit.
Book Aragorn was eagerly awaiting his time to be king. Movie Aragorn didn't want anything to do with it.
Book, Denethor is being controlled by Sauron via the Palantir, in the movies he's a brutish, cruel, tyrant.