The Horror film Exorcist is based on the same name novel of William Peter Blatty.
William Peter Blatty was studying at the Jesuitical Georgetown University, read in a Newspaper about the Mt Rainer exorcism case. Mt Rainer was a fourteen years old boy who was possessed by the Devil. Finally Ctaholic Priests exorcised him after several failed attempts. Later Church announced that the Mt Rainer case was not a demonic possession case but it was possibly a case of hysteria with mild telekinetic effects. Blatty had a different opinion the case of Mt Rainer was evidence for the existence of God and Satan.
According to “Cinema pf the Occult new age, satanism, Wicca and spiritual films" (Carol L. Fry) the book was inspired by the exorcism case of Roland Doe, which involves the demonic possesion of young girl and her’s mother’s desperate effort to unchain the life of hers daughter from demonic spirits by asking two Priests to conduct an exorcism.
William Peter Blatty, believed that a book about demonic possession open new gates to the unknown and will travel readers imagination into new territories. He started to search sources for his book.
Firstly, William Peter Blatty approached the Catholic Priests who conducted the exorcist of Mt Rainer. There was a negative answer from Priests, the family of Mt Rainer was not willing to reveal information about the nightmare of possession.
How the Exorcist was born.
One of the Priests who had conducted the exorcism Father Bowdern, was willing to help William Peter Blatty to research the possible novel based on the real case of Mt Raines, he proposed to Blatty the change the gender of the victim - the gender of demonic possessed youth from boy to girl in an effort to protect sensitive data information of the Rainer family.
The fisrt draft of the novel was finished in the summer of 1970. Blatty showed his work to Chris McNeil and she took it to producer Lew Grade, but there was no interest for film rights.
The Film Rights of The Exorcist
Harper and Row bought the rights of The Exorcist, and finally the rights of the book were bought by Warner Bros for $641.000 and Blatty took the position of producer and also wrote the firs draft of screenplay.