From American Cinematographer, Demonic Convergence by David E. Williams (August 2008)
Heat and lighting became key issues throughout the exorcism sequence. In the story, the paranormal activities leave the bedroom frighteningly frigid, and since the authenticity-obsessed Friedkin wanted to see the performers’ breath vapor, the set was built in a refrigerated room ironically dubbed "The Cocoon." The crew kept the room at a temperature of about -20°F, making heavy coats a necessity for anyone working in the space.
"We did some tests with the temperature at about 25 degrees, and you could see some breath, but it wasn’t really enough," Roizman says. "When the lights were turned on, their heat warmed the room so quickly that we couldn’t even get a single take. The breath showed up fine at zero, but Friedkin wanted the actors to really feel the cold because he thought it would help their acting. An actor on his knees for 20 minutes at minus 20 degrees is really going to feel the cold."
[The room was ostensibly illuminated by a pair of small table lamps set on either side of Regan’s four-poster bed. To ensure that the breath vapor would read correctly, Roizman painstakingly created a backlight effect for each actor while trying to stay true to his source-lighting approach.]
"This created a problem because our sources were right next to the bed, and [the priests] were always facing the light. The challenge was to get the backlight on the breath while keeping it off everything else. With the actors moving all the time, it became difficult to hide the backlight and keep it off them; my gaffer, Dick Quinlan, was usually sitting on the floor behind Max and Jason, handholding an inky-dink with a snoot on it and just getting the light on the vapor. He did an amazing job."
- Owen Roizman, ASC