In the movie, Val Kilmer (as John Holmes) wears a chain around his neck that holds a ring. The ring was the actual wedding band that John Holmes gave his wife, Sharon, when they got married in 1965. Sharon loaned it to Val to give him good luck on the movie. When the movie was over, Sharon then gave the ring to Dawn Schiller.
Both Josh Lucas and Val Kilmer spent the night in the actual Wonderland house on the anniversary of the murders.
The actual police investigation tape of the crime scene is included as a special feature in the DVD release of the film.
Christina Applegate grew up near the site of the Wonderland murders and remembers seeing the bloody mattresses on the street, which led to her agreeing to take a relatively small role in the film.
In the climactic murder scene from the John Holmes perspective, there are several single-frame shots of the actual corpses from the 1981 LAPD crime scene photographs.