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.
Exorcist II: The Heretic
Flashback with the Other Darrin: Blair agreed to reprise her role of Regan Mac Neil 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.