The character of the disturbed killer Curt Duncan was based on a college acquaintance of director Fred Walton who somehow could just enter a room and automatically make others in the room uncomfortable. Yet Walton felt enough sympathy for this person that the character of Duncan is treated with a fair amount of sensitivity.
Fred Walton originally shot this film as a short entitled 'The Sitter', which was essentially the opening 20 minutes of 'When A Stranger Calls'. However after the huge success of Halloween (1978) Walton saw the potential of expanding the short into a full-length feature. The script was then expanded into a feature length film about the pursuit of villain Curt Duncan.
Star Tony Beckley was terminally ill throughout the production, because of this he did not fit the description of the killer but Fred Walton refused to recast him as they were good friends. He later passed away just after the principal photography was shot. Director Fred Walton dedicated the film's 1993 sequel 'When A Stranger Calls Back' to the memory of Beckley.
The phone number of the house where the character Jill Johnson (played by Carol Kane) is babysitting, (555-2368) is the same house phone number used in Jamie Lee Curtis's house in the movie Forever Young (1992) starring Mel Gibson.
Throughout the opening segment, director Fred Walton gradually increased the feeling of suspense by making each subsequent phone call ring a touch louder than the previous one. They escalate from eerie to jarring and finally infuriating.
Carol Kane plays a babysitter who is harassed and stalked by a serial killer, ironically she herself would go on to play the killer in another slasher movie, Office Killer (1997), where she stalked and attacked an office full of employees.
The trivia items below may give away important plot points.
The film's legendary opening 20 minutes of Jill getting harassed on the phone by a killer only to find out that the calls are coming from inside the house were inspired by a similar sequence in the original Black Christmas (1974). Both films took inspiration from the urban legend "The Babysitter and the Man Upstairs", though this film is more of a direct adaptation of the myth.
The Classification and Rating Administration had originally voted unanimously for a PG rating (5 years before the PG-13 rating was available for use). However, CARA chair Richard Heffner then viewed the film and called the board for further discussion to consider voting for an R rating instead. Although the theme of a film could potentially be accommodated within a PG rating, Heffner argued that this film's treatment of its theme was too unsettling for most parents to want it to be freely available to unaccompanied children (as the killer murders two children with his bare hands in the first act even though it happened off-screen). A majority vote was then received to assign the film its R rating.
As this movie was made because of Halloween (1978)'s success, it bears many similarities to the movie. Both movies start out with an infamous murder scene ending with the killer being apprehended and sent to a mental institution. After a time gap of several years, the killer breaks out of the institution, with someone who has known him since his arrest (In Halloween (1978), it's the killer's former psychiatrist, in this film, it's the killer's arresting officer) pursuing him with the intent of killing him rather than apprehending him, despite objections from coworkers. The killer begins to stalk a female (in this film, it's not only the female lead, unlike Halloween (1978)). The killer narrowly avoids getting caught by the male lead several times. Both movies also end with a showdown between the female lead and the killer, with the killer beginning to overpower the female lead, but the male lead suddenly appears and guns down the killer. (However, in this film, the killer dies from the gunshot wound. In Halloween (1978), the killer survives)