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
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.
Regan MacNeil: You're going to die up there.
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.
Question: What was the significance of the father being an absentee dad? Did that make Reagan more vulnerable to being possessed?
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.
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.
TonyPH