[Linda Blair] mouthed everything, but we also recorded her own voice as a guide track and for a while I thought we might use that recording. When I started the picture, I thought, “I’m just going to get a good, ballsy, masculine voice to do this thing.” But it occurred to me that it would be much more believable if I could get a female voice that had some masculinity to it. Most of her voice is replaced by Mercedes McCambridge’s, but some of the voice is her own. The stuff that was most effective was recorded in sync to her own dialogue, line for line. All Linda Blair did was mouth the words as best she could. Mercedes McCambridge, who smokes heavily, was able to speak in that emphysemic voice and get that wonderful wheezy sound. We would experiment. She would swallow three raw eggs and drink some Jack Daniel’s and then we had her tightly tied to a chair. It sounds like she has three or four screaming animals in her throat. We recorded that very close up and then made a loop out of it. After we had dubbed the girl’s voice I felt there was something wrong. It occurred to me that I had to keep the demonic presence alive, even when it wasn’t talking, and that’s when we decided to put the looped wheeze in.
The media makes up shit that you can’t believe. They said after making The Exorcist Linda Blair was in a mental hospital or something. She was a delightful little twelve-year-old girl, and every time we’d do a take of the most monstrous things imaginable the prop man would hand her a milkshake. I made every scene a game with her.
I knew that the only way I could make this movie was if I had a child who was able somehow to grasp and deal with this horrible stuff that had to be performed. I really thought I might never find such a person. We had casting directors look at thousands of women across the country, starting at age twelve. Then we started looking for sixteen-year-old young women who looked younger. We couldn’t find anyone, and I seriously thought it wouldn’t be possible to make the film. Then in comes this eleven-year-old girl with her mother. I ask her the same questions I asked the others. I said, “Do you know what this story is about?” She said, “Yes, I read the book. It’s about a little girl who’s possessed by the devil and she does a lot of bad things.” I said, “Like what? What sort of bad things?” She said, “Well, she pushes a man out a window and she hits her mother in the face and she masturbates with a crucifix.” I said, “What?” I’d never heard that from an eleven-year-old. I said, “What does that mean?” She said, “What?” I said, “What does ‘masturbate’ mean?” She said, “It’s like jerking off, isn’t it?” I said, “Uh-huh.” Then I said, “Have you ever jerked off?” and she said, “Sure. Haven’t you?” I said, “You’re hired.”
Excerpt from The Great Moviemakers: The Next Generation, compiled and edited by George Stevens, Jr.