William Peter Blatty wanted the film to be titled simply "Legion", just like his novel of the same name. The producers, however, wanted the title to be "The Exorcist III" for commercial reasons. Blatty even tried to convince them to alter it to "The Exorcist 1990" in order to distance it from Exorcist II: The Heretic (1977), which he despised, but had to settle for "The Exorcist III" notwithstanding.
William Peter Blatty offered directorial responsibilities to John Carpenter, who liked his script, but backed out when it became clear that Blatty really wanted to direct the movie himself.
William Peter Blatty made the film based on his novel "Legion," which was published in 1983. Morgan Creek decided to call it "The Exorcist III: Legion" even though the screenplay and novel featured no exorcisms whatsoever. After principal photography was completed, the misleading nature of the title was noticed, and the producers determined that additional scenes needed to be written and then added into the film in order to make it a more recognizable sequel of The Exorcist (1973). The last third of the movie had to be entirely redone, with the insertion of a new character, Father Morning, and of an exorcism scene that cost nearly four million dollars.
On the climactic exorcism scene, William Peter Blatty later said, "It's all right, but it's utterly unnecessary, and it changes the character of the piece." Although at the time, Blatty told the press that he was happy to re-shoot the film's ending and have the story climax with a frenzy of special effects, the truth is that this compromise was forced on him, against his wishes: "The original story that I sold (Morgan Creek), and that I shot, ended with Kinderman blowing away Patient X. There was no exorcism. But it was a Mexican stand-off between me and the studio. I was entitled to one preview, then they could go and do what they wanted with the picture. They gave me a preview, but it was the lowest end preview audience I have ever seen in my life. They dragged in zombies from Haiti to watch this film. It was unbelievable. But I decided, better I should do it than anyone else. I foolishly thought: I can do a good exorcism, I'll turn this pig's ear into a silk purse. So I did it."
William Peter Blatty had hoped to recover the deleted footage from the Morgan Creek vaults, so that he might re-assemble the original cut of the film, which he said was "rather different" from what was released. Unfortunately for Blatty, and for the fans who had been clamoring for such a release, the footage was lost for years. On June 28, 2007, Blatty's wife sent the following comment to a fan site: "My husband tells me that it is Morgan Creek's claim that they have lost all the footage, including an alternate opening scene in which Kinderman views the body of Karras in the morgue, right after his fall down the steps. What a shame." However, the missing footage was finally found in 2016, and Blatty's original version was restored shortly before his death the next year, in 2017.
Patrick Ewing: As the Angel of Death next to Father Dyer in the dream sequence. Like William Peter Blatty, Ewing was a graduate of Georgetown University.
C. Everett Koop: As Himself in The Tombs, a local Georgetown restaurant where Kinderman has lunch with Father Dyer.
Larry King: As Himself in The Tombs, a local Georgetown restaurant where Kinderman has lunch with Father Dyer.