I started the research a long time ago, before I had children, so I used to stay at work really, really, late and use some of my firm’s research tools to search for “demons,” “possession,” etc in databases. The halls would be dark. I would be the only one of the 15th floor. And I would read these interviews with M. Scott Peck, the famous self-help author, about how he assisted in exorcisms (he was an Army doctor) and how he saw a possessed woman transform before his eyes into the shape of a snake. The image stuck with me and I used it in one scene.
A buddy of mine was working for Warner Brothers on web promotion for the re-release of The Exorcist. I shot him an email to forward to William Peter Blatty, the author of the novel and screenplay, asking what sources he had used for The Exorcist. I got a two-line email back—ALL CAPS—containing only the names of two books, one from the nineteenth century, the other a collection of essays by Jesuits. I had to hunt them down in libraries—old hardcovers. The first one freaked me out so badly I had to put it down after three pages. I was in the Cooper Union library, on was a bright, hot summer afternoon in the middle of the city, but my hands were shaking, and I ran out of there like, well, like the devil was behind me.
The Exorcist, by the way, is supposed to be based on an actual incident that happened in the DC area while Blatty was a college student there. Reading about that is pretty chilling. Check out five chapters on the topic(!): http://www.strangemag.com/exorcistpage1.html
Another key inspiration was an anecdote from a book about witchcraft in which a man is beset by an imp. I view this as an archetypal encounter with a wicked spirit; one that helped me shape the encounters between George and his Friend.
Finally, I spent an inordinate amount of time querying Catholic scholars on a popular Catholic website. Take a tour, and explore:
