The mask of the monster suit was altered considerably. When Ray Corrigan was fitted for the suit, the mask was initially too tight. Paul Blaisdell, who made the suit, had to remove and rebuild the monster's lower jaw so the mask would fit better. Unfortunately, Corrigan's chin stuck out through the opening made in the mask. Blaisdell made up his chin to look like the monster's tongue. The mask's original eyes (large and catlike, a Blaisdell trademark) were also removed; the eyes you see behind the mask are actually Corrigan's.
Inside the "It" costume was actor, stuntman and frequent gorilla performer Ray "Crash" Corrigan. This would be Corrigan's final film, ending a film career of over 25 years.
For the main title score, Bert Shefter and Paul Sawtell (I) simply recycled their main title score from Kronos (1957) and changed the opening bars.
According to director John Carpenter, filming took only six days. Carpenter didn't direct this film but he was acting as a co-host in an interview with Robert Osborne prior to it being shown on the Turner Classic Movies channel.
The final battle between the monster and crew is being shown at the drive-in during the Bryan Adams' video "Summer of '69".