A new documentary offers an intimate portrait of William Friedkin, director of The Exorcist, who says his horror classic about demonic possession is actually about the mystery of faith

Spirit possession could be a neurological phenomenon – a violent change in metabolism – whose clinical symptoms resemble those of epilepsy or hysteria. Spasmodic convulsions, muscular contortions and even orgasms are not infrequent. The phenomenon is common to Afro-American animist religions born of slavery and common also to charismatic Christianity, the movement of spiritual renewal that began in Kansas in 1901 with Pentecostalism. The Church of God and other modern Apostolic sectarian offshoots may attribute a holiness to trances, paroxysms and glossolalia or the “gift of tongues”.

To the Catholic Church, however, glossolalia is more likely to be seen as a spiritual aberration. The clergy’s official handbook on exorcism, the Rituale Romanum, cites it as evidence of demonic bewitchment.

William Peter Blatty’s 1971 novel, The Exorcist, based on a real-life Roman Catholic exorcism case in Maryland in 1949, was a best-seller for 15 months before it was brought to the screen by director William Friedkin.