Two of the notorious gay bars featured in the film - Mine Shaft and Eagle's Nest - eventually barred William Friedkin.
Karen Allen was never shown a complete script before she worked on this film. Director William Friedkin deliberately kept her in the dark, since her character Nancy wasn't supposed to be aware of what was happening to Al Pacino's character as he explored the gay underworld.
The film is based on a series of murders of gay men that took place between 1962-79 in New York City.
Richard Gere was William Friedkin's first choice for the lead, and was even signed for the lead role and was very excited for the project. "I think he would have been wonderful," says Friedkin, "because he had a strange, ambiguous quality about him." Pacino got a hold of the script and decided he wanted the role, and as one of the biggest and most acclaimed actors at the time (Serpico, 1973; The Godfather Part II, 1974; Dog Day Afternoon, 1975) it was decided that he would be the better bet.
For research, William Friedkin worked with members of the Mafia, which owned most of New York City's gay bars at the time.