The Exorcist novel — on which the film is based on — was initially a failure when it was first published in 1971. After the book’s author, William Peter Blatty, made a chance TV appearance, the book shot to No. 1 on The New York Times best-seller list.

While promoting the book, Blatty was pre-interviewed as a possible guest on late-night talk show The Dick Cavett Show. However, he was told by the interviewer that it would be unlikely he would be picked as a guest.

As luck would have it, Blatty was asked to come in at the last minute after one of the guests suddenly dropped out. While at the taping, another guest dropped out and he was given the remaining time (nearly 45 minutes) to talk about the book.

After Blatty’s appearance on the show, the book shot to the top of The New York Times best-seller list.

Source: Los Angeles Times

Via etsy.com

Prior to the book becoming a best-seller, every Hollywood studio rejected Blatty’s screenplay for the film.

After the book became a best-seller, it caught the attention of then-Warner Bros. studio head John Calley, who bought the rights.

Blatty not only wrote the script, but also served as the film's producer.

Source: Los Angeles Times

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Blatty's novel was inspired by the alleged 1949 case of demonic possession and exorcism of a young boy in Maryland.

The 1949 case involved a 13-year-old boy who underwent 30 Catholic exorcisms to free him of demonic possession. The church gave the boy the pseudonym of "Roland Doe."

Doe has no recollection of his possession.

In 2012, the film's director, William Friedkin, told a panel during a Q&A at the ArcLight that Doe recently retired from working at NASA.

Via behindtheexorcist.com