During shooting, the crew (and specifically Álex de la Iglesia) received various death threats from so-called satanist groups.
Early in the film the priest hands a piece of paper to José María reading "-Napalm Dez -Iron Maiden -Hace de Cé". This is his way of pronouncing metal bands Napalm Death and AC/DC.
Santiago Segura, who plays hardcore metalhead "José María" claims not to be interested at all in Heavy Metal. Instead, he prefers Frank Sinatra.
After the movie's moderate success in the US, Álex de la Iglesia sold the rights to an American remake (which would be directed by de la Iglesia himself) and was offered the director's chair in Alien - A Ressurreição (1997). Though neither project came to fruition, de la Iglesia still directed his next movie in the US, Perdita Durango (1997).
Most rituals showed in the films are real satanist rituals and weren't altered at all.