Movies “based on” books. Oh, as a confirmed SFF geek, where do I begin? So many, many bad movies made from good books. Okay, I get it: every movie can’t achieve the utter worshipful perfection of Peter Jackson’s THE LORD OF THE RINGS. But some could definitely try harder is all I’m saying.
Let’s start with the inimitable Andre Norton. In 1959 she published THE BEAST MASTER, book one in the tale of Hosteen Storm, a Navajo who was telepathic with several genetically-modified animals. They all get sent to an alien planet and the usual Norton hijinks ensue. Good, classic Andre Norton, am I right? 
Then why, pray tell me, oh why did the screenwriters for the 1982 THE BEASTMASTER even suggest that they “based” their story on Norton’s book? I mean, what the who? Now suddenly we have the barbarian Dar—brilliantly portrayed by Marc Singer’s pecs—who as a fetus gets magically transferred from his mama’s womb into a cow—dazzling special FX here as the cow’s belly swells—and grows up into the aforementioned bundle of pecs. Then there’s a “story” about how he meets and bonds with a big cat, a hawk, and a pair of ferrets—the ferrets being clearly the best actors in the entire movie, as I still remember their names: Kodo and Podo. There’s a big bad guy—Rip Torn in his best scenery-chewing mode—a noble black guy and a fur bikini filled with Tanya Roberts. Don’t get me wrong, I loved the movie, but it was obviously made by someone who had: 1. Never read Norton’s book, and 2. Only knew in the very vaguest fashion what constituted a good fantasy movie. Hint: it’s not pecs and a fur bikini. 
However, any and all movies would be improved by the addition of a pair of ferrets—fact. TITANIC, for instance, badly needed ferrets.
Oh, yeah, and Andre Norton insisted her name be removed from the credits. Surprising, huh?
Next in this litany of infamy is the pair of movies based on John W. Campbell-writing-as-Don-A.-Stuart’s novella, “Who Goes There?” First was 1951’s THE THING FROM ANOTHER WORLD, starring pre-Gunsmoke James Arness as a vegetable alien—no, he’s not a confirmed plant eater, he’s an actual plant. The novella and the movie share a location—a claustrophobic base in the Antarctic—and a basic premise—alien ship frozen in ice inadvertently destroyed but inhabitant saved—but skips over most of the shapeshifting bit. Still, it does have a lovely sense of fear and danger and I adored it as a kid, as I uncritically adored anything faintly SFF.
In 1982, John Carpenter directed THE THING, with a bearded Kurt Russell and a team of actors familiar from TV. This one was much, much closer to the original novella and, even though I was no longer in grammar school, I loved it. This time, as in the story, the alien can replicate others, leading to a growing sense of paranoia. Who’d human, and who’s not?
Great Carpenter-esque scenes of decapitated heads sprouting legs to run away from flamethrowers—honestly, does it get any better than that? And my favorite scene of all: a doctor is doing CPR and it goes terribly, terribly wrong, not only for the patient…but for the doctor. Watch it and see.
Oh, and there’s a suitably depressing ending that leaves the viewer up in the air about who is still infected and who survives. Or doesn’t.
And finally, THE PUPPET MASTERS. Heinlein’s 1951 book is one of my favorites of his, full of thinly-disguised Commie obsession of that time period, but with some seriously kick-ass characters. And the movie wasn’t bad, don’t get me wrong. I mean, all Donald Sutherland needs to do is stand there and I’m agog. Hey, I even drop what I’m doing to listen—with varying amounts of drool—to that lovely, lovely voice pitch orange juice.
And Sutherland is not miscast as the head of a secret organization who’s the only one to realize Earth is being invaded and Earthlings being controlled by soul-sucking aliens that look like slugs with rather fetching whip-tails. In the book, though, one of his operators is his son, Sam, and another is a brilliant badass red-haired woman named Mary—who has an interesting backstory that plays into the denouement.
In the movie, Sam and Mary are played by, well, actors that don’t fit my image of the roles is all I’m going to say. Story doesn’t suck too much either.
Heinlein’s books deserve good movies. The closest I’ve seen is PREDESTINATION, based on his “All You Zombies.” It gets the twisty-turny-wibbly-wobbly time travel bits down. Go watch it if you haven’t seen it.
And watch the others too. After all, they are SFF…after a fashion.