Paul Ehlers's son Jonathan Ehlers was born during the shooting of the movie. Paul received the news that his wife was in labor while filming a scene in full Madman Marz make-up. Paul went immediately to the hospital and held his newborn son for the first time while still wearing some of his Marz make-up and overalls covered with fake blood.
According to Paul Ehlers, the fake feet he wore as part of his Madman Marz costume were very slippery and nearly caused him to fall out of the tree in the shot where Madman watches the campers in the woods.
Originally, 'Madman' was supposed to be based on the Cropsy killer legend, but when word reached the production that The Burning (1981) was being based on the same legend, the script was re-written.
The crew had to paint the leaves of trees and bushes green on location, because the film was being shot in winter, but was supposed to be set in warmer weather.
Tom Candela mimics the 'stairs' for the scene where Richie goes down into the Madman's basement, because the room he was entering was actually a closet at the location house.