Despite playing a high school age couple, Julia Duffy was about ten years older than Jimmy McNichol. Duffy was approximately 30 years old at the time of filming.
Though no one involved in the film's creative process has ever given an on-record interview about the genesis for the movie, it apparently began as a novel. A book released around the same time as the movie under the title "Butcher Baker Nightmare Maker" is far more complex than a simple novelization. It includes vivid physical descriptions of the characters that often differ greatly from the actors' onscreen appearance, as well as in-depth backstories for several characters, including secondary characters who receive little screen time. The book also leaves it a mystery until far later in the narrative as to what happened to Billy's parents, and also continues on past the movie's ending, wrapping up the stories of several characters whose fates are not addressed in the film's epilogue.
Reportedly star Susan Tyrell never saw the film, believing it to be "awful," until an interview in 2008. After watching it however she said she enjoyed it.
Jimmy McNichol performed his own stunt for the bit where Billy falls down the attic stairs.