The 1968 film Rosemary's Baby brought the concept of Satan into the forefront of horror movies, a trend that continues to this day, although director Roman Polanski designed the film to have more than one interpretation. Produced by William Castle, the movie featured Mia Farrow and John Cassavetes as a couple who move into an apartment building with a history. Then things get weird. Things were weird behind the scenes, too.
According to Farrow, actor Sidney Blackmer (who played coven leader Roman Castevet) once said on set “No good will come of all this ‘Hail Satan’ business,” and apparently he wasn’t the only one who thought so. William Castle later became convinced the film was cursed. Shortly after production he suffered gallstones to such a severe extent that he required surgery. As he recovered from that illness, Rosemary’s Baby composer Krzysztof Komeda suffered an accidental fall that led to a coma and, eventually, his death. Then, in the summer of 1969, actress Sharon Tate—Polanski’s wife—was infamously murdered by the Manson Family. For Castle, it all added up.
"The story of Rosemary's Baby was happening in real life. Witches, all of them, were casting their spell, and I was becoming one of the principal players,” he later recalled.
Learn more of what went into the production of Rosemary's Baby at Mental Floss.