The Exorcist, screenwriter William Peter Blatty's adaptation of his own popular 1971 novel, draws from two primary real-life sources. The first pertains to the character of Father Merrin (Max von Sydow). While in Beirut, prior to writing The Exorcist, Blatty met a British archaeologist named Gerald Lankester Harding. As Blatty soon learned, Harding had excavated the caves where the Dead Sea Scrolls were discovered.
The second, more intriguing influence behind The Exorcist connects to actress Linda Blair's possessed character, 12-year-old Regan McNeil, herself. She's loosely based on a young Maryland boy, who, in 1949, was given an exorcism by famed Catholic exorcist Father Walter Halloran.