John Carpenter's 1982 sci-fi/horror classic The Thing is a mostly faithful adaptation of its book source material, but here's what did get changed. Carpenter's The Thing is often called a remake of director Christian Nyby's 1951 film The Thing from Another World, and while that makes for an acceptable shorthand, it's really not entirely true. Their stories are certainly very similar, but Carpenter clearly went back to author John W. Campbell's novella Who Goes There? for inspiration, as his film is much closer to the book than the 1951 version.
For instance, The Thing from Another World's monster is much different than that found in Campbell's book or Carpenter's film. While the creature is still thawed out from a block of ice - this time in Alaska, not Antarctica - what emerges is a mostly humanoid, plant-based alien that feeds on blood to survive. This choice was partially due to special effects at the time not really making Campbell's monster possible to realize properly, and while it worked fine for audiences at the time, Carpenter was able to realize Campbell's intent onscreen.
With that said, it would be much more accurate to refer to Carpenter's The Thing as a re-adaptation of the book, and not a remake of the 1951 movie. As faithful as it is though, some aspects of the plot and characters did get altered, and here's what was changed.
The first major change John Carpenter's The Thing makes to the novella Who Goes There? is that the setting is updated from its publication year of 1938 to the then-present of 1982. That makes sense, as it takes a lot of effort and money to make a believable setting so far in the past. Another big change is the addition of the Norwegian camp and its crew, who were almost all killed and assimilated by The Thing before the American crew's story begins. In the book, the Americans discover The Thing's ship, accidentally destroying it in the process.
The Thing movie also greatly cuts down on the American crew, reducing them from nearly 40 to only 12. That was a great choice, as having that many characters floating around onscreen would get unwieldy and hard to follow, plus it allows the base to be smaller and more claustrophobic. Most of the characters in the film appear in the book, serving the same roles, outside of Childs, who was invented for the movie. In one of the biggest plot changes, the primary method of killing a Thing in the book is electrocution, as opposed to the "kill it with fire" solution onscreen.
In a change with interesting implications, the book implies that an assimilated human may not be aware they're a Thing until the creature takes control. The movie makes it pretty clear that Things know full well they aren't human, and scheme to ensure their own survival. The Thing movie's famous blood test scene is actually very close to how it's written, with MacReady speaking some dialogue verbatim. However, the test in the book leads to a different, happier ending, in which Blair is revealed to be the final Thing, he's disposed of, and humanity wins.
