Trivia: Much of the ending was restructured and reshot to make the climax more action packed. In fact, the entire exorcism scene and the character Father Morning was added in lengthy and rather expensive reshoots. In the original ending, Kinderman simply shoots Patient X/Father Karras/The Gemini Killer to stop the madness, then leaves the room. The studio felt the ending was a little too short and anti-climactic, and had William Peter Blatty write a new climax to end the film with a bang.
Trivia: Brad Dourif has stated that part of the reason Karras' physical form morphs into the Gemini Killer whenever the killer "takes over" his body was because actor Jason Miller (who plays Karras) was struggling with severe alcohol addiction, and it had begun to affect his memory, making it difficult for him to memorize the Gemini Killer's long diatribes. They kept Miller's scenes short and with minimal dialogue, while Dourif (playing the killer) handled all of the lengthy speeches and monologues.
Trivia: Director William Peter Blatty (who also wrote the original film and the novel it was based on) absolutely hated the much-reviled second film, "Exorcist II: The Heretic." Thus he went out of his way to make sure this film contained no references to it at all, and didn't consider it canonical to this film. (Similar to how "Highlander 3" ignores the second film or how "Halloween H20" ignores the events of "Halloween" 4-6).
Trivia: Director William Peter Blatty's original cut of the film was believed to be lost, as the studio could not locate the footage. However, an old videotape containing much of the lost footage was found in 2016, and Blatty was able to splice the VHS-quality footage into the movie to assemble an impromptu "director's cut" shortly before his death. The "director's cut" was finally released on Blu-Ray through boutique company Shout Factory in late 2016.
Trivia: At one point, the Gemini Killer says the line "it's child's play," and the film cuts to a little red-haired kid. This is a cute little in-joke, as the Gemini Killer is played by Brad Dourif, who voices the killer-doll Chucky (a doll with red hair) in the "Child's Play" horror movie series.
Trivia: Director William Peter Blatty wanted to title the film "Legion" after his novel the script was based on, while the studio (understandably) wanted to call it "The Exorcist III" for brand-recognition reasons. Blatty absolutely hated "Exorcist II: The Heretic" and felt that the title "The Exorcist III" would suggest this movie followed that one, so he countered by suggesting they title the film "The Exorcist 1990." The studio eventually won out and went with "The Exorcist III."
Trivia: The film features some well-known public figures and performers in strange cameos or small early roles. For example, model Fabio, basketball player/coach Patrick Ewing and future megastar Samuel L. Jackson all appear in the trippy dream sequence. (Albeit Jackson's voice is dubbed.) And legendary television/radio host Larry King is also briefly shown eating lunch during the diner scene early in the film. (The camera briefly lingers on him for about two seconds during the establishing shots).
