Firstly, it's not an old movie - Gone With The Wind is an old movie. So is Stagecoach.
The author of the book and I went to Jesuits schools, though not together, but we both read from a book called "Satan, Begone!" which are the memoirs of a real-life exorcist from the Catholic Church. Many of the episodes from that book made it into "The Exorcist" such as the speaking in backwards Latin, and the naming of ancient demons. I was ready for the movie,, then, as I had read the memoirs, and Mr. Blatty's novel from which the movie was adapted.