In the scene in Stone's apartment where they are discussing their names, Alastair Duncan is actually laughing. Because Rutger Hauer held it together so well, it was decided to keep it in the movie.
Gary Scott Thompson's original 1988 speculative script was titled "Pentagram". Set in Los Angeles, California, it involved a ritualistic serial killer who murders five people every five years during the last quarter-century, leaving a pentagram symbol after each kill. It was an action, buddy-cop, horror movie with occult overtones. The script got Thompson more jobs before it was finally picked up by the head of Challenge Films, Laura Gregory, and Production Manager, Susan Nicoletti, who got director Tony Maylam involved. All three told Thompson the story was too similar to Pentagramm - Die Macht des Bösen (1990). Thompson re-wrote the script and re-titled it "Black Tide". This version was set in London, England, which was half under water due to global warming. Rutger Hauer told Thompson to re-write it to make it more physical, with more focus on psychic connection between Stone and the creature.
The film spawned the term "The Rutger Hauer Diet", namely anxiety, coffee, and chocolate.
Despite decent word of mouth and an exciting advertising campaign, the movie underperformed at the US box-office due to the fact that it opened during the weekend of the Los Angeles, California riots.
Gary Scott Thompson wrote the script with Harrison Ford in mind for the lead role but was happy when Rutger Hauer was cast. Hauer loved the script, which is why he wanted to do the movie.