Some movie adaptations of books are really good, some despite the changes and others because of them. Some things work way better on screen than on page, and vice versa!
Jurassic Park is my go-to example of my favorite book-to-screen adaptation. I love the book, and I love the movie, and they do different things that work well for their mediums. My favorite change is with the kids: in the book, the two kids are an older brother (about 9) and a younger sister (about 5). The little girl doesn’t do much except for being a source of stress for Alan Grant to have to protect. It works, she’s a cute little kid in danger, but she’s barely a character. The little boy obsessed with dinosaurs is a real character. In the movie, the boy (Tim) stays about 9 years old but the girl (Lex) becomes the older sister, about 12 years old, and this is probably partially about child actor capabilities but also it allowed her to be a character. She’s a computer nerd who likes indoor stuff, dislikes nature, and doesn’t care about dinosaurs, contrasting her with Tim and making it hard for Grant to connect with her in an opposite way he’s awkward with Tim. And then, at the end, when the kids get access to the park computer system and turn the power back on, in the book it’s Tim desperately guessing, which adds a certain sense of tension and urgency; but in the movie, it puts Lex finally in her element, leading to the famous line “It’s a UNIX system… I know this!” And it’s a satisfying narrative payoff for her out-of-place survival stress the whole rest of the movie. (Also it allows the girl Something To Do! Which in the book she barely got because she was five!)
There are other changes: Ian Malcolm, John Hammond, and Henry Wu survive the movie which they didn’t in the book, and in exchange the lawyer who survived the book got eaten by a dinosaur. They cut the boat subplot that the lawyer came in clutch for, which made sense because it would be kind of distracting for the movie’s tense survival stress focus, so the lawyer now had little narrative value except to abandon the kids and get eaten by a dinosaur (forcing Grant to step up and protect the kids despite being Awkward With Kids.) It was narratively tight and Worked. Ian Malcolm surviving, meanwhile, was probably because he was played by Jeff Goldblum and they wanted to keep him around. However this choice was so popular that Michael Crichton retconned his death in the book sequel haha.
Hammond and Wu also both have different tones in the movie than book: movie!Hammond is well-meaning but so convinced of his own vision he refuses to listen to any of the experts he hired, and Wu, his head scientist, is portrayed as… if not an enabler, at least so smugly confident in his own control over these creatures he made that he doesn’t think there should be any problems. In the book, Hammond is more uncomfortably stubborn and business-minded, and Wu is portrayed as arguing with him a lot, trying to convince Hammond to let him genetically engineer these dinosaurs to be slower and dumber to be safer, and Hammond rebuffing him because that’s not authentic, and Wu basically tearing his hair out because none of this is authentic!!! … but Hammond is his boss and paying him Lots Of Money so he guesses he’ll do what Hammond wants. He gets eaten by a Velociraptor for this. Wu is barely a character in the movie, but I would have loved to see a scene where he admits to Grant that he wanted to make the raptors slower and stupider but Hammond won’t let him because “authenticity,” and Grant has conflicted feelings about this, but Hammond grumpily rebuffs this idea and moves them on. I think it could have slotted in well, though it would have muddied the themes of the human expectations and business and control vs. nature defying their desires and tight control, especially because the movie understandably cut a lot of the chaos theory discussions that formed the thematic core of the book. I get why they did it the way they did.
The movie made quite a few changes and still works very well as both a movie and an adaptation, and all of the changes range from understandable to Actively Good. Good adaptations of books to movies exist, and they always involve changing or cutting things! But the intentionality and thought are the important parts. And some exhibit this more than others.
