In the film's final scene in Brooksville Cemetery, a tombstone with the name "Daily" is seen over Andy's shoulder. The same tombstone appears in another one of Bob Clark's horror films, Children Shouldn't Play with Dead Things (1972).
Richard Backus was cast as Andy in the film because he was able to create a silent stare of intense hatred for the casting agent. Backus' creepy facial expression came into play frequently in it.
Portions of Carl Zittrer's music score for this film were later reused and expanded upon for another one of Bob Clark's horror films, Black Christmas (1974).
For the film's opening scene in Vietnam, there was additional footage shot of a traumatized Andy walking through the jungle among the bloody carnage. The footage was cut in favor of a quicker beginning to the story. There was also supposed to be a similar scene later in it where Andy walks through the streets at night and comes across a blind veteran sitting on a porch. This scene was also cut for reasons of time.
Alan Ormsby: the writer of the film's screenplay, who played the man who tells Charlie that Doc Allman has been killed. Ormsby was married to Anya Ormsby (who played Cathy Brooks in the film) at the time. Their young son made a cameo appearance in it as the crying boy in the scene where Andy kills the dog.