Not always. I know Disney’s The Little Mermaid is actually quite a bit better than the original tale, getting the intended theme of self-sacrifice down FAR better than the original tale did (not to mention made the lead character a lot more sympathetic if you ask me. Sorry, but 1. I find it extremely irritating that God would fail to give a species that wasn’t just sentient, but sapient as well, at the very least something similar to an immortal soul if not an immortal soul itself, and 2., I just found the mermaid in that story to be very unlikeable, especially when many of her actions have dire consequences for her family, she left for a very selfish reason regarding pursuing a soul, and unlike Ariel, she doesn’t even ATTEMPT to make amends for what her actions caused her family even when she committed suicide. And she gets a soul? Had it been me who wrote the book and in Andersen’s shoes, I’d probably have her cast down to Hell for what she did.). I guess I could also cite Jaws and Forrest Gump, even though they weren’t exactly my cup of tea either in book format or film format.
That all being said, we can definitely agree that the book version was MUCH better than the crappy film adaptation in the case of Starship Troopers. Heck, it wasn’t even intended to be an adaptation of Starship Troopers the novel, it was originally a completely unrelated film called Bug Hunt on Outpost Nine.