Background Music: Fans of Greek rebetika music love the scene where Fr. Karras visits his mom. True to the book, she's got the radio on, tuned to a Greek-language station (possibly WNYE). As Karras comes in, we hear Rita Sakellariou's "Istoria Mou, Amartia Mou" (My Story, My Sin). Later as he leaves his mother sleeping in her chair, the radio is still playing softly, Yannis Kalartsis' "Paramythaki Mou" (My Tale). The lyrics to this one are by Lefteris Papadopoulos, an award-winning journalist as well as composer, who requested compensation some years later as he was destitute.
Shown Their Work: Boy howdy. William Blatty dug really deep, finding literature about supposed epidemics of possession in the past as well as writings pro and con Psychic Powers and evidence for and against supernatural occurrence. He based much of the details on the legend of the "Devils of Loudon", where nuns lifted up their skirts and yelled "Fuck me!", and a priest who did exorcisms was said to have become possessed himself. The exorcisms at Loudon were a public spectacle, held in the town square! The stories told by the Jeane Dixon Expy at Chris' party are all real, as is the tale Fr. Karras tells Kinderman about the man who was convinced he was a werewolf. However, that last one was mostly made up by the Church to rationalize the torture and execution of a man and his daughter for becoming Protestants. Loudon was similarly politically motivated.
Write What You Know: Chris MacNeil is based on William Peter Blatty's friend Shirley MacLaine, and her marital situation on MacLaine's then-husband having left for Europe determined to not live in his wife's shadow. What's weird, is that nobody even cares to try and contact him because of his daughter's worsening health to the risk of DEATH.
This caused Sachiko MacLaine a bit of trouble, as numerous people had heard Blatty had based Chris on Shirley and assumed that Sachiko had been sick or possessed.
Billing Displacement: Max von Sydow only appears in the movie during the prologue and the last twenty minutes, but is billed second, despite Jason Miller and Linda Blair's characters being the main focus of the film. The reason for this error is likely because von Sydow was already well known, and Miller and Blair were screen newcomers.
Special mention must go to the shot where Ellen Burstyn is "thrown" by Regan across the room and her back hits the tallboy. Burstyn was forcibly thrown and dragged with a wire harness, giving her a spinal injury that's plagued her her whole life. Her scream of pain is real.
The real reason Father Dyer is shaking while he's administering last rites to Father Karras is because William Friedkin slapped his actor just before the take.
Linda Blair had to endure the freezing cold bedroom set wearing only thin nightgowns.
Feelies: The US theatrical release of the film gave free barf bags to audience members.
Enforced Method Acting: John Boorman filmed the scene on Regan's balcony, which isn't a set, knowing full well he had no way of catching her if she fell. Her screams of fear are real.
Flashback with the Other Darrin: Blair agreed to reprise her role of Regan MacNeil under the condition that she wouldn't wear the demon make-up, so a double was used for the brief flashback scenes depicting a demonic Regan.
Old Shame: For both director John Boorman and Linda Blair, Blair even going as far as calling it "one of the big disappointments of my career."
Troubled Production: Well let's see, director Boorman contracting a dose of San Joaquin Valley Fever which cancelled production for over a month, footage being over-saturated and necessitating re-shoots, the rapid deaths of locusts imported from England for the film’s climactic scenes (2500 locusts were shipped in, and died at a rate of 100 a day); original film editor John Merritt quitting the production (replaced by Tom Priestley); and stars Kitty Winn and Louise Fletcher both suffering from gall bladder infections, to say nothing of the constant script re-writes, sometimes even as they were shooting the film.
What Could Have Been: William O'Malley was contacted to reprise his role as Father Joseph Dyer from the first film. However, O'Malley was busy and could not take up the part, and the character of Father Dyer was changed to Father Philip Lamont. Jon Voight, David Carradine, Jack Nicholson and Christopher Walken all were considered or offered the part of Father Lamont, who John Boorman initially conceived as a younger priest in awe of Father Merrin's writings. Eventually the choice was made to age the character, and Richard Burton was signed for the role.
The Exorcist III
Actor Allusion: Lee Richardson states that his favorite movie is The Fly. About a year earlier, he starred in the sequel.
Executive Meddling: The ending was reshot at the insistence of the producers, because they wanted an actual exorcism in the film. They also forced William Peter Blatty to title the film The Exorcist III instead of his preferred title, Legion.
Test audiences wanted someone from the original film to appear, so they hired Jason Miller and reshot the Patient X scenes. In this case, it worked, since there was a clear division between The Gemini Killer and Father Karras.
The Other Darrin: Lee J. Cobb died in 1976, so the part of William F. Kinderman was recasted with George C. Scott.
A bit of a meta Brick Joke as Scott also plays another character previously played by Cobb: Juror #3 of 12 Angry Men.
Exorcist: The Beginning
Executive Meddling: Took place to a savage degree with Paul Schrader's original cut, which was entirely junked by the studio and refilmed from scratch by Renny Harlin.