The "Exorcist steps," 97 stone steps in Georgetown, were padded with 1/2"-thick rubber to film the death of Father Damien Karras.
The stuntman tumbled down the stairs twice.
Georgetown University students charged people around $5 each to watch the stunt from the rooftops.
Eileen Deitz was a double for Linda Blair to film some of the scenes, including the crucifix-masturbation scene.
The refrigerated bedroom set was cooled with four air conditioners and temperatures would plunge to around 30 to 40 below zero.
It was so cold that perspiration would freeze on some of the cast and crew.
On one occasion the air was saturated with moisture resulting in a thin layer of snow falling on the set before the crew arrived for filming.
After filming, Friedkin brought production to 666 Fifth Av.
The demon voice was initially supposed to be Linda Blair's.
After 150 hours of work on sound recording in pre-production, Friedkin rejected it.
He called on the voice of Mercedes McCambridge who was fed raw eggs, alcohol and cigarettes, and then was strapped to a chair, causing her to form the unusual vocals.
Gonzalo Gavira was called on to create many of the special sound effects after Friedkin recalled his work from the movie, Topo, El (1971).
One of the more memorable sounds, the 360-degree turning of Regan's head, was actually made by twisting a sound crew member's old leather wallet in front of a mike.
The network TV version was edited by William Friedkin who shot a replacement insert of the Virgin Mary Statue showing her face painted like a harlot (instead of the more obscene version in the theatrical film).
The looping of the Demon voice to remove the profanity was also done by Friedkin himself because of his unwillingness to work with Mercedes McCambride again.
Lines like "Your mother sucks co*** in hell, Karras" and "Shove it up your a** you faggot" were redubbed by Friedkin to be "Your mother still rots in hell" and "Shut your face you faggot.
" By and large, this network TV version is not used for TV and cable showings today.
The nurse who comes into Dr. Taney's office after the arteriogram is actress Linda Blair's mother.
The Prospect Avenue apartment where the story takes place was once inhabited by the author, William Peter Blatty, while he was a student at Georgetown University.
Continuity: The cigarette that Chris stomps out in the park is back in her hand a moment later.
Continuity: Position of the planchette on the Ouija board.
Continuity: Near the end of the picture, Father Karras' stole mysteriously disappears after he walks into Regan's room -- he is suddenly wearing a blue pullover.
Minutes later, downstairs, he's wearing his stole again, and when he returns upstairs to check on Father Merrin, he's wearing the pullover again.
Revealing mistakes: Rubber mats are visible on the steps for the famous fall down the staircase scene.
Continuity: The white towel around Father Karras' neck.
Revealing mistakes: When Regan is floating over her bed, the lines that lift her are clearly visible.