I generally prefer books to their movie adaptations if I've read the books first, and movies if I watch them first. For example, I love the Lord of the Rings and the Harry Potter books and am not a big fan of the movies (Fellowship of the Ring was good; the other ones changed the characters and ignored the themes too much), but I watched V for Vendetta and Watchmen before reading the comic books and enjoyed both of the movies. I particularly don't get the complaints in the case of Watchmen; in most respects it's the most faithful adaptation of ANY work I've seen, to the point of using the exact same dialogue, scenes that look identical to ones depicted in the book, and even including music depicted in the comic book in the soundtrack. It's a deeply disturbing movie, sure - because the book's a deeply disturbing book, and is meant to be.
The one adaptation I can think of that arguably improves on the original work is - apologies to Jane Austen - Emma Thompson & Ang Lee's Sense and Sensibility. The male characters are far more fleshed out, the relationships are more developed; in the book, there's really no reason to like Edward Ferrars or to think he deserve Elinor's affection. In addition, some parts are simply more realistic and emotionally powerful. In particular - in the book, Marianne becomes deathly ill from taking a few evening walks in a damp garden, something that may be in line with 19th-century views on female frailty but which is hardly credible to the modern reader. In the movie, she walks a long distance is a driving rainstorm to look at Willoughby's house, which is very much in line with her sentimental character and also makes serious illness somewhat more realistic.
This may be because Austen wrote the book as a character study and social satire, while the movie is mainly a romance.
The Hunger Games books are great. The movies tried to capture it, but even some of the dialog didn't come through clearly. The first Twilight book was so slow I could barley get through it. They really did improve the story and that's saying something based on how slow the movie is.
I just happen to write an article series about Disney movies which are based on books. Concerning the Harry Potter movies: I give the first one a lot of credit. For book readers it might look bloated, but it did have to create a whole world for the people who didn't know the book. The explanation of Quidditch aso might have dragged a little bit on, but it was necessary, and the final confrontation was very well done IMHO. The second movie - I actually liked it, it was very whimsical, but this fit the tone of the book. The third movie - I think it is the best movie of the series, but it's frustrating that it came so close to being a really good adaptation, too, and then they just had to throw in stuff like the idiotic talking heads. This and the reveal concerning Sirius was too rushed. Then the fourth movie came and the series good downhill. This movie just has a pacing problem, because it spends hours on stuff like the stupid dragon walking over the castle, but rushes through the really meaningful scenes. The graveyard scene kind of makes up for it, though. The fifth was okay again, though I think the movie makers should have taken some time for Harry and Ginny (but that was an ongoing problem, that they liked Hermione too much, effectively ruining her and the other characters in the last movies), but then the last three - unlike some people I liked the last two books, too, but the movies are totally missing the point. They concentrate so much on the action stuff and not enough on the meaningful dialogues. Just burning down the borrow and cutting out the "I not afraid when you are with me" line in the sixth movie showed how much they didn't get it. And the last battle...it should have been epic, it should have been dark, instead we got a watered down version and four people stupidly stabbing at a snake.
I always say: if a movie adaptation does not make you want to read the source material, it is good on its own. For example, I love The Mask, but do not want to read the original comics, whether they are good or bad; I just don't care.
Solaris 1972 - the Tarkovsky version. Complete garbage. Tarkovsky did not understand Lem at all. Beautiful imagery aside, this movie has the wrong casting, wrong (or missing) characterization, wrong plot and wrong everything else. I'm not against a movie director changing the story and putting a creative spin on it (I love Gankutsuou, for example) but this was really bad.
Solaris 1968 - the almost unknown black-and-white TV film, actually pretty damn good. It's sometimes incomplete in terms of awesome moments that were in the book, but they replaced those moments with something that gets the point across and allows other, "dependent" scenes to stay logical. The casting is great. Etush was a suprising choice for Snaut/Snow, but he fits the role damn well. Also, there are a few film-only scenes that fit the story so much I almost thought they were in the book until I remembered they weren't. I would recommend both the book and this adaptation.
Solaris 2002 - the Soderbergh version. I haven't seen this one, but it was stated to be a remake of Tarkovsky, not a re-adaptation of Lem, so I'm guessing it's not very good. The Cliff Martinez OST is fantastic, though.
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It makes perfect sense. Raw entertainment and density can often collide with each other.