I’ve often said (as have many others) that it’s better to take a good short story and flesh it out with details and texture than to take a large novel and pare it down to fit into the 2-3 hour time slot of a movie. Especially for science fiction or fantasy. They’ve been able to get away with trying to film novels in many cases (the Harry potter books, for instance), but failed utterly in others (Dune, where they cut out huge gaping chunks, and made the story incomprehensible).
The problem is that, in many cases when they do this, they end up clipping away so much of the story even so, and padding it rather than fleshing it out, that it becomes an entirely different animal, and you wonder why they bothered using the original story to start out with. I understand the needs of a cinematic story are different from those of the printed page, but there are plenty of counter-examples (especialy in non-genre fiction) where they did an admirable job of adaptation that it makes it glaringly obvious that they’ve been doing serious damage to SF, especially because the movie versions are far worse.
it has been done well, I have to admit – 2001 is far more than “The Sentinel” was. the day the Earth stood till goes way beyond what was in “Farewell to the Master”, and the 1951 Nyby/Hawks the Thing throws out most of Campbell’s “Who Goes There?”.
But in other cases they’ve screwed it up. “The Space Frame” was a clever little story wholly lost in Invasion of the Saucer Men. “Mimic” was buried and padded out of recognition in Mimic. They exhausted virtually all of “We Can Remember it for you Wholesale” in Total Recall within the first few minutes (padding it, I’ve maintained with stuff taken from Robert Sheckley’s “The Status Civilization” and a dash of Burroughs’ “A Princess of Mars”). It looks as if The Last Mimzy is going the same route with “Mimsy were the Borogoves”, but we’ll see.
Any other cases? Any non-genre cases where the film wanders off the plot of its nominal source?