The actress (Jenny Powell) playing the character of Mary Ann Mathews was originally a stripper hired in to act as a stand-in. Joel Schumacher gave her the part of the victim on the 8mm film as she had a suitably "haunted" look about her.
The plot is extremely similar to that of Hardcore (1979) in which a man (George C. Scott) hires a private detective to track down his daughter after she runs away and he sees her in a 8MM porno movie. The plot is also very similar to a book called "The Cutting Room" by Louise Welsh in which an estate auctioneer finds disturbing photos among a client's porno collection and endeavors to find out if the girl in the pictures was actually killed.
The film's production encountered concerns regarding the dark subject matter, and the studio asked Andrew Kevin Walker to lighten the film's tone. With Joel Schumacher as director, Walker felt a rewrite would no longer be needed. But as it turned out, Schumacher supported the studio and made changes of his own, leading to a much-publicized fallout between the two, with Walker virtually disowning the film and walking away from the set. He refused to even watch the film.
The car belonging to Eddie Poole is a Stutz Blackhawk, a very rare high-status car of the 70's and 80's, owned by such luminaries as Elvis Presley and Frank Sinatra. (Elvis scooped Sinatra by offering almost double what Sinatra had deposited on a Stutz.)
The Aphex Twin song, "Come to Daddy", is played twice in 8MM. The first time it is played as the theme song to a hardcore porn flick, and the second time it is actually played as the background music to 8MM, upon Nicolas Cage's entering of the "bad guy's" house. Also, the music video for "Come to Daddy" is being played on a small TV in Dino Velvet's office.