To create the effect of the words "help me" appearing on Regan's torso, special effects artist Dick Smith created a foam latex replica of actress Linda Blair's stomach. The words were then written on the latex using cleaning fluid. This caused the words to "rise up" out of the latex due to a chemical reaction. The words were then filmed. As the film ran, the words were heated with a blow dryer, causing them to deflate. When the film was run backwards, it appeared as though the words were rising out of Regan's skin.
For the scene where Regan's throat expands, a simple bladder was used beneath a latex appliance.
To achieve the effect of cold breath in Regan's bedroom, the bedroom set (20 feet on each side, with a 20 foot high ceiling) was insulated with 8 inches of fiberglass. Four cooling units, designed for use in meatpacking warehouses, hung from the soundstage rafters and cooled the set down to about 10 degrees Fahrenheit. These $50,000 air conditioning units kept breaking down, causing extensive repair bills and delaying the shooting of the scenes for days on end. Even when they worked correctly, delays were common because the camera lights heated the room so much that shooting would have to be delayed so that the air could return to below freezing. It was so cold on the set that the crew had to wear parkas and gloves, and at time frost would appear on props and furniture.
There were three separate beds built to do three separate movements in the film. Each bed was made out of steel tubing, because the effects crew knew they would take a severe beating during production.
- One version of the bed had a hydraulic brace fitted to a steel plate under the mattress. To make Regan appear to be sitting up and falling down in bed at high speed, actress Linda Blair was strapped tightly to the brace using a leather harness, and her costume placed over the brace and harness. The hydraulic brace forced Blair upright and back down every second. Her neck and head were allowed to flop around, as were her arms, to make the effect appear fluid. Run at slightly higher speed, the effect appears very violent. (Blair injured her upper back and neck during the filming of this effect, although the severity wasn't apparent until years later. She suffers from terrible spine problems to this day.)
- For the "leaping bed" effect, a beam was attached to the steel headboard. This beam went through the wall behind the bed, where it was attached to some counterweights. With the bed slightly off the ground (supported by that headboard beam), crew on the other side of the wall then pulled on the counterweights -- causing the bed to rock violently side-to-side. (Essentially, the effect was "done in reverse": The bed was already in the air, and the effects team behind the wall were trying to pull it back down to the floor.)
- For the slowly levitating bed effect, a steel beam was attached to the headboard and inserted through the wall. Behind the scenes, a forklift slowly raised and lowered the bed.
To achieve the effects of furniture moving across the floor, the bedroom set was actually built on gimbels. When tilted, the set literally threw people or furniture across the room.
The bedroom set also had a false ceiling. It was not attached to the walls, and could be literally pulled apart by winches behind the scenes. When Father Merrin first hits possessed Regan with holy water, the winches would activate and the ceiling would crack open.
The the head-turning? A radio-controlled mechanical body double. To ensure that the thing looked real, effects supervisor Marcel Vercoutere rode around Manhattan in the back seat of a taxicab with the thing in the back seat. Whenever they'd stop at a red light, he'd activate the head -- and if the people in the car behind screamed, he knew the effect worked.
But the film is best known for the power-vomiting scene.
To create this effect, 30-year-old stunt double Eileen Dietz was made up to look like Linda Blair. Smith created flattened tubes out of heat-formed plexiglass that ran from the corners of Dietz's mouth, across her cheeks, and behind her head. These connected to tubes running down her back and beneath the bed to pumps. In Dietz's mouth was a black tube (similar to the device a dentist uses to keep a patient's mouth open) that connected to the plexiglass at the corners of her mouth. A nozzle close to the center of Dietz's mouth spewed the "vomit". Dietz could barely swallow or close her mouth. The "vomit" was actually Andersen's pea soup, mixed with a little wet oatmeal. The crew tried Campbell's soup, but didn't like the effect. The effect was filmed.
The same pea soup mixture was loaded into a projector and aimed at actor Jason Miller ("Father Karras") to get his reaction shot. Friedkin told Miller the mixture would hit him in the chest, but the aim was wrong and it hit Miller directly in the face. Miller's reaction was so good, Friedkin kept it in the final film.
Once the film got into editing, Friedkin became unhappy with the look of the vomiting scene. So the footage of Dietz was junked! Instead, Vercoutere quietly filmed new footage of a thicker pea soup mixture in his garage. A shot of Linda Blair miming the vomit action was also filmed, and the two images composited together. In stills, you can see that the vomit does not come from the center of Blair's mouth but from just inside the cheek -- because the angles of the shots were every so slightly off.