Rosemary's Baby is a 1968 American psychological horror film written and directed by Roman Polanski, based on the bestselling 1967 novel of the same name by Ira Levin. The cast features Mia Farrow, John Cassavetes, Ruth Gordon, Sidney Blackmer, Maurice Evans, Ralph Bellamy, Angela Dorian, Clay Tanner, and, in his feature film debut, Charles Grodin. The film chronicles the story of a pregnant woman who suspects that an evil cult wants to take her baby for use in their rituals. Rosemary's Baby earned almost universal acclaim from film critics and won numerous nominations and awards.

In Rosemary's Baby: A Retrospective, a featurette on the DVD release of the film, screenwriter/director Roman Polanski, Paramount Pictures executive Robert Evans, and production designer Richard Sylbert reminisce at length about the production. Evans recalled William Castle brought him the galley proofs of the book and asked him to purchase the film rights even before Random House released the publication. The studio head recognized the commercial potential of the project and agreed with the stipulation that Castle, who had a reputation for low-budget horror films, could produce but not direct the film adaptation. He makes a cameo appearance as the man at the phone booth waiting for Mia Farrow to finish her call.  Evans admired Polanski's European films and hoped he could convince him to make his American debut with Rosemary's Baby. He knew the director was a ski buff who was anxious to make a film with the sport as its basis, so he sent him the script for Downhill Racer along with the galleys for Rosemary. Polanski read the latter book non-stop through the night and called Evans the following morning to tell him he thought Rosemary was the more interesting project, and would like the opportunity to write as well as direct it.  The script was modeled very closely on the original novel and incorporated large sections of the novel's dialogue and details, so much so that nearly every line of dialogue was taken from the novel's text. Author Ira Levin claimed that during a scene in which Guy mentions wanting to buy a particular shirt advertised in The New Yorker, Polanski was unable to find the specific issue with the shirt advertised and phoned Levin for help. Levin, who had assumed while writing that any given issue of The New Yorker would contain an ad for men's shirts, admitted that he had made it up.

Using a quote from the above article, answer the following question: What was his inspiration for the story?
The script was modeled very closely on the original novel and incorporated large sections of the novel's dialogue and details,