Chris consults a number of physicians, but Dr. Klein and his associates find nothing physiologically wrong with her daughter, despite Regan undergoing a battery of diagnostic tests.

The Rev. William O'Malley is best known for his portrayal of Father Dyer in The Exorcist, for which he was also a technical advisor. According to one source, he was the first Catholic priest to portray a priest in a commercial motion picture.

The Exorcist's most iconic image is of the moment that Father Merrin arrives at the MacNeil residence and, illuminated by a street lamp, looks up at the home. This image was inspired by René Magritte's Empire of Light paintings.

Regan turns twelve at the beginning of the film.

"Keep away," the demon warns the exorcist. "The sow is mine."

Dr. Klein suggests a prescription of Ritalin for Regan, apparently believing she suffers from ADHD (Attention-Deficit/Hyperactivity Disorder).

While shooting a scene where Regan throws her from the bed, Ellen Burstyn permanently injured her coccyx. That was her real scream of pain that was used in the film.

While saying Mass, Father Karras says "Remember also, O Lord, Thy servant, Mary Karras, who has gone before us with the sign of faith, and sleeps the sleep of peace."

"It's a symptom of a type of disturbance in the chemical-electrical activity of the brain," the doctor explains. "In the case of your daughter, in the temporal lobe -- it's up here -- in the lateral part of the brain. It's rare, but it does cause bizarre hallucinations."

Father Damien Karras meets Chris for the first time on the Key Bridge walkway, where she asks him how to get an exorcism. He is against the idea, but he agrees to see Regan as a psychiatrist.