Continuity mistake: After the exorcism, Regan's wrists are tied together, but are then later seen tied to the bed.
The Exorcist (1973)
121 continuity mistakes - chronological order
Directed by: William Friedkin
Starring: Max von Sydow, Ellen Burstyn, Lee J. Cobb, Linda Blair, Jason Miller
Visible crew/equipment: Near the beginning of the movie, the old priest is in Northern Iraq and you can see the shadow of a microphone on his hat.
Demon: What an excellent day for an exorcism.
Father Damien Karras: You would like that?
Demon: Intensely.
Father Damien Karras: But wouldn't that drive you out of Regan?
Demon: It would bring us together.
Father Damien Karras: You and Regan?
Demon: You and us.
Trivia: In the scene where Regan's mother is supernaturally blasted away from the bedside, she is being yanked by the crew by a length of rope. After dozens of takes director William Friedkin was still unhappy with the look of the shot and ordered the crew to haul her more fiercely. The scream in the shot that made the cut was of genuine pain and required no dubbing for effect.
More questions & answers from The Exorcist
Answer: It was indeed Pazuzu, he wanted Regan to believe he was some friendly entity to gain her trust. Thus the Captain Howdy facade.