From American Cinematographer, Demonic Convergence by David E. Williams (August 2008)
One particular special makeup effect relied on a simple camera trick. After opening Regan’s nightshirt, Karras is shocked to see the words "Help Me" appear as raised letters on her heaving stomach. Roizman reveals that the girl’s belly was actually a foam latex prosthetic; prior to shooting, a reactive chemical was painted onto it to create the raised letters. With his camera running in reverse, the cinematographer shot a close-up on the two words, which gradually receded as the chemical evaporated. When the footage was cut into the final film, the plea seemed to mysteriously arise from the girl’s flesh.