This was Faye Dunaway's first movie.
The basic plot of this movie bears a striking resemblance to that of a minor British comedy of about eight years earlier, "Too Many Crooks", and was used again many years later for another American movie, "Ruthless People".
During this film's shoot, Robert Walker Jr. worked on another project at the same time. The second week of filming, he did double duty filming an episode of The Time Tunnel (1966), where he played Billy the Kid.
British actor Brian Bedford was set for "The Happening" but parted company with director Silverstein and was replaced. This worked out well for the actor because it allowed him to do the big budget "Grand Prix."
A variation of the track "The Fuzz", played in the movie, is used as the opening of many TV news programs, including Jornal Nacional (1969), which is still in use after more than 50 years.