The film was originally completed in late 1988 and was one of the last films completed by distributor New World Pictures when they filed for Chapter 11 bankruptcy. The film's trailer was actually shown before early 1989 theatrical releases. Instead of going direct to video, Warlock was picked up and released in theatres by Trimark Pictures in 1991. Its box office success lead to a sequel, "Warlock: The Armageddon" in which Trimark also released theatrically.
When Julian Sands received the script, he assumed it was in the mold of the then-popular slasher movies, so it sat around for a while before he got around to reading it. Once he finally decided to give it a once-over, he was captivated and quickly became excited about the film.
In 1995, Saskatchewan teenager Sandy Charles and an unnamed 8 year-old accomplice murdered a 7 year-old boy, cut strips of flesh from his body, boiled and drank it in hopes that, as in the film, it would give them the ability to fly. Charles was found not guilty by reason of insanity but was sentenced to stay at the high security Regional Psychiatric Centre in Saskatoon, where he has primarily resided since shortly after the murder occurred.
Although the makeup tests had been finalized and approved, on the day of shooting Lori Singer decided she didn't want to don the elaborate prosthetics for her transformation into a 40 year-old, and she later refused to sport some of the makeup designed for her 60 year-old transformation.
Screenwriter David Twohy's original conception was that the Warlock was a good-natured man forced flee the 17th century to avoid religious persecution only to find himself similarly persecuted in the 20th century. After working on the script for 2 months, he realized it didn't work and decided to make the character a villain.
The trivia items below may give away important plot points.
The scene in the theatrical trailer involving the Warlock as the satanic messiah was cut after negative reactions from test screenings. The channeler's death was originally much more violent. The scene involved a topless Mary Woronov being frozen after the devil's eyeballs emerged from her breasts. The Warlock then stomps her to pieces to free them. After the wrap up, the scene was redone as what remains in the film.
The warlock's goal is to bring together all three parts of a book that will allow him to learn the "true" name of God, and undo all of Creation by speaking the name backward. In many spiritualistic pagan religions, evil spirits were said to have personal names kept from all others, for learning the true name of a spirit would allow an entity to gain control over it.