Day of the Jackal was pretty faithful to Frederick Forsyth’s novel, except for some cutting (I assume to keep the length short enough and the pacing good), which is all to the good. I was very disappointed with the way they changed The ODESSA File a couple of years later (although, in retrospect, it could’ve been far worse).
That’s pretty surprising. I’ve noticed that espionage novels and science fiction/fantasy generally have the most changes between novel and movie – look at the various Jason Bourne movies, or James Bond moviees, or Ludlum’s The Osterman Weekend or MacLean’s Ice Station Zebra. As far as fantastic fiction, the 19th cdentury authors – Wells, Verne, Poe, Haggard, and (by courtesy) Lovecraft get changed the most.
Surprisingly faithful SF/Fantasy:
**The Call of Cthulhu
The Whisperer in Darkness
When Worlds Collide
The Man Who Could Work MIracles
The Thing** (1982 John Carpenter Version with Bill Lancaster script – up until they find the craft near the end)
The Day of the TRiffids (1981 TV version only – The Day of the Triffids (1981 TV series) - Wikipedia )
I think The Man Who Fell to Earth might be pretty true to the original novel, but I confess that I haven’t read the novel.
The TV movie Judge Dee and the Monastery Murders was pretty true to Robert H. van Gulik’s The Haunted Monastery. And I wish they’d done more of the Judge Dee novels.
The Hawksworth productions BBC/WGBH Sherlock Holmes adaptations (starring Jeremy Brett) were extremkely faithful to the times and the stories, and the changes were all to the good, at least until mthe end, when Brett’s illness and the need for more material lead to over-padded scripts with not enough Holmes in them. But the early ones were superb, the best Holmes adaptations I’;ve seen.