In the original script, the skull which flies out of the freezer was not included - it was added by Lucio Fulci. He later remarked that he felt it was one of the most clever things he had come up with, and was the only thing about the movie he was truly proud of.
Like many Italian movies shot for the international market, most of the cast delivered their lines in English, but were then re-dubbed for a potential American release anyway. Only a few members of the cast actually spoke Italian.
Lucio Fulci's cut of the film was approximately 70 minutes, before he had to leave the production due to a stroke. Producer Franco Gaudenzi asked second unit director Bruno Mattei and writer Claudio Fragasso to take over the director's chair. Mattei had been in the Philippines at the time filming Mission suicide (1988). He could not get the main actors back to the Philippines, so their scenes were turned into a subplot. Fulci's footage was cut down to around 50 minutes and about 40 minutes of new footage were added. Mattei claimed that he shot about 40% of the film, including all of the scenes in the beginning and all of the scenes with the soldiers dressed in white anti-contamination suits.
Blue Heart's supposed radio set list (briefly seen in the shot where his sunglasses are sitting on the desk in front of him) is in fact just a piece of printed paper with only "Play Beatles Song" written on it in red pen.
Marina Loi auditioned for the part of Patricia, but it wound up going to Beatrice Ring. However, Lucio Fulci took a liking to her and gave her the role of Carole.
Claudio Fragasso: The writer, and the second director, Bruno Mattei, have brief cameos as soldiers in the crematorium scene.