The screenwriters added the plot element that Lomax was Milton's son, and that Milton could produce the Antichrist, neither of which are in the novel. Hackford cited the films "Rosemary's Baby" and "The Omen" as influences, and both had explored the Antichrist mythology. Another change from the novel was converting the book's lesbian client to the male molester Lloyd Gettys, avoiding undertones of homophobia. In an early version of the screenplay, the "Better to reign in Hell than serve in Heaven" quotation is given to Milton rather than Lomax.