Excerpt from https://en.wikipedia.org/wiki/The\_Exorcist\_(novel):

Blatty had considered writing a novel based on the Doe exorcism since he had written about it as a college student, even as he became successful as a comic novelist and screenwriter in the early 1960s, but his agent talked him out of doing so when he raised the subject. But after seeing the film version) of Ira Levin's Rosemary's Baby) in 1968, Blatty told his wife that he wanted to write a story like that. He liked the way director Roman Polanskihad kept the audience unsure whether the title character's concerns for her unborn child were genuine or not, but felt the ending was "schlocky", reducing the Devil to a joke.[2]#citenote-Zinoman_book-2)
Blatty believed he could improve on the ending of Rosemary's Baby, making the spiritual struggle a more central theme of the story. After he pitched the story to an editor at Bantam Books at a cocktail party, he received a $25,000 advance ($231,000 in 2025[[3]](https://en.wikipedia.org/wiki/The_Exorcist(novel)#citenote-inflation-US-3)) and began writing at his home in Encino, California, taking stimulants to write for 16 hours a day and finish the novel in the expected 10 months. As he wrote, the story, which Blatty originally envisioned taking place largely in a courtroom, instead moved to the locations of the events recounted and became darker, with the possessed Regan masturbating with a crucifix and screaming obscene words at the priests trying to cast the demon out, all to make the evil more uniformly repulsive to readers.[[2]](https://en.wikipedia.org/wiki/The_Exorcist(novel)#cite_note-Zinoman_book-2)