(at around 1h 20 mins) When Regan projectile vomits at Father Karras, the vomit was intended to hit Jason Miller in the chest, but the plastic tubing misfired, hitting him in the face. His reaction of shock and disgust while wiping away the vomit is genuine, and Miller admitted in interviews that he was very angered by this mistake.
Jason Miller had a major verbal confrontation with William Friedkin after the director fired a gun near his ear to get an authentic reaction from him. He told Friedkin that he was an actor, and that he didn't need a gun to act surprised or startled.
On the first day of rehearsing the exorcism sequence, Linda Blair's delivery of her foul-mouthed dialogue so disturbed the gentlemanly Max von Sydow that he actually forgot his lines.
Due to death threats against Linda Blair from religious zealots who believed the film "glorified Satan," Warner Bros. had bodyguards protecting her for six months after the film's release.
According to director William Friedkin, the subliminal shots of the white faced demon are actually rejected makeup tests for Regan's possessed appearance played by actress Eileen Dietz, who also stunt double for Linda Blair.
Elinore Blair: (at around 48 mins) The nurse who comes into Dr. Taney's office after the arteriogram is Linda Blair's mother.
William Peter Blatty: (at around 12 mins) The author/producer appears in the film, similarly to real life, as a movie producer. He can be seen debating a scene with Burke Dennings on the set of Chris's film.