One shoot, at an abandoned hospital, fell on Friday the 13th. The crew became convinced the location was haunted as throughout the evening the lights and the elevator turned on and off by themselves.
Along with Joe Dante's The Howling this film pioneered the trend of air-bladder special effects makeup. For Michael's transformation scene small plastic sacks (often condoms or balloons) would be embedded into the layers of makeup and face castings. Later while filming these sacks would be inflated through tubes and it would help to give the appearance of the skins distortion. The same technique became popular in the genre throughout the 80's being used in such horror films as Demons, Fright Night, and Evil Dead II.
Star Paul Clemens was very enthusiastic about having the role of Michael MacCleary because he was an avid fan of the horror genre. Clemens would even enjoy the extensive makeup work that would take hours to apply to him.