Directed by: William Friedkin
Starring: Max von Sydow, Ellen Burstyn, Lee J. Cobb, Linda Blair, Jason Miller
Other mistake: When Karras throws himself out of the window, why is a man who needs a cane running full speed down those steep steps? It makes no sense since people use canes for balance.
Suggested correction: Some people have a cane as a fashion statement. Also, it's possible the cane helps but is not totally necessary, certainly able to be ignored when rushing to an emergency.
Revealing mistake: You can see wires in the scene where Regan levitates above her bed in the exorcism scene, lifting her up. The way they pull on her dress is also visible.
Demon: I'm not Regan.
Father Damien Karras: Well, then let's introduce ourselves. I'm Damien Karras.
Demon: And I'm the Devil. Now kindly undo these straps.
Father Damien Karras: If you're the Devil, why not make the straps disappear?
Demon: That's much too vulgar a display of power, Karras.
Trivia: The vomit that Linda Blair spewed was actually split-pea soup. Specifically, the effects crew decided to use Anderson's split-pea soup because Campbell's split-pea soup wasn't green enough.
Question: What was the significance of the father being an absentee dad? Did that make Reagan more vulnerable to being possessed?
Answer: The demon wants everyone to lose hope for the girl and give up on her. Her father, having already flown the coop, just makes this goal that much easier.
Answer: It's possible it made her more vulnerable, but the mother, being a big movie star with an adolescent child would probably be divorced at this stage. It was more likely a literary choice by the author. Fewer characters make for a less complicated plot and eliminates the need to add an additional character who may not serve the story in a meaningful way.