The main character was originally called "John Cassavetes" and was in fact going to be played by actor-director John Cassavetes. When he withdrew from the film due to a scheduling conflict, the character's name was changed to "John Cassellis" and Robert Forster was cast in the role.
In the psychedelic nightclub sequence, the band seen performing on the stage is The Litter, a Minneapolis-based group. However, in the original release version, their music is not heard; instead, we hear a piece by Frank Zappa's 'Mothers of Invention'.
When the film was released on video, Paramount was sued by the copyright holders of the song "Merry-Go-Round". Under their 1969 agreement, Paramount had rights to the song for showing the film in theatres and on television. Paramount argued that video release was the same as television broadcast. The courts ruled that the copyright holder in 1969 could not have considered videocassettes to be like television broadcast, as home videocassettes were not invented.
The line "Watch out, Haskell, it's real!" was actually dubbed in after the shooting. It was supposedly what Haskell Wexler was thinking to himself and he wanted to include it. Verna Bloom literally stands out in the film because of her yellow outfit. Bloom said she picked the outfit because she thought her character, a woman of modest means, would wear something like that. It just happened to contrast with what everyone else was wearing, so you notice her.
Some of the background intercom chatter playing during the first scenes shown of the television station that John Cassellis works at is the same intercom chatter used in George Lucas' student film Electronic Labyrinth THX 1138 4EB (1967). Paul Golding, who served as an editorial consultant for this film, had also been a collaborator on some of Lucas' other student films.