The character of Colonel Kurtz in the novel was changed to "Colonel Curtis" for the movie, so the audience would not think it was a reference to the Apocalypse Now (1979) character, which it is in the novel.
The trivia items below may give away important plot points.
Stephen King wrote the novel while recovering from a near-fatal accident. Because of his injuries, he wrote the novel in longhand. The accident is reflected in the story, particularly a graphic scene in which a vehicle runs down a major character.
Mr. Grey's death in the film was completely different from the book. In the movie, he is killed by Duddits in the water supply room. In the book, using their powers, both Henry and Duddits smother him to death with a pillow.