Friedkin says Blatty set out to write a non-fiction account of an exorcism that happened to a 13-year-old boy at a psychiatric clinic in 1949, but had to dramatize the story when it became too difficult to get specific details of what happened.
Those who did speak to Blatty requested that the character be changed to a girl, to help protect the identity of the boy who actually experienced the possession. Friedkin says that child grew up having no memory of the incident, and went on to an otherwise stable life. Friedkin said he recently retired from a long career at, of all places, NASA.
The filmmaker never met that man, but spoke with family members who described telekinetic activity surrounding the child during his apparent possession.
“The family was Lutheran and they went through all the stages you see in the film: they went to doctors, clinics, and finally went back to their own pastor in the Lutheran church, who recommended they see a priest,” Friedkin said.
It’s not that anyone in the medical profession actually believed a demon was the problem, but they thought the power of suggestion might help the boy if he thought it was a true possession. A priest named Father William Bowdern reportedly performed the ceremony, with a younger priest named William Halloran assisting.
The incident was even mentioned in Halloran’s 2005 obituary in The Washington Post.
When The Exorcist was in the early stages of production, Friedkin met with the Rev. Robert J. Henle, then president of Georgetown, who secretly passed him an old red folder with Halloran’s diaries and other eyewitness accounts of the true-life exorcism.