Guillermo del Toro met with Universal (who later bought out the film's U.S. distributor, October Films) in late '93, where they told him they wanted to buy the rights to this film so they could remake it. del Toro's response was "Who wants to see Jack Lemmon lick blood off a bathroom floor?".
The alchemist at the beginning of the movie is named Fulcanelli, which was the pseudonym of a famous french alchemist of the late 19th/early 20th century, who mysteriously disappeared in the 1940s and whose real name and identity has never been known.
The film went over budget from the original $1,5 million to $2 million (the highest budget for a Mexican movie at the time). Guillermo del Toro himself got the half million through loans and bank debts. In order to complete the film, changes had to be made, among those changes were Ron Perlman, who agreed to a heavy salary cut. Perlman and del Toro has been good friends ever since, working together frequently.
In an interview included on the Criterion edition of this movie, Ron Perlman talks about how Angel was meant to speak Spanish fluently. Ron Perlman tried this, but Guillermo del Toro found his reading to be completely unusable. So, the character was changed to an expatriated American who so hates being in Mexico, that what little Spanish he speaks is deliberately spoken poorly.
The two De La Guardia characters were deliberately intended to be somewhat unreal, like comic book characters. Guillermo del Toro explains in his commentary that he did this as a sort of revenge against Hollywood films that having Mexican characters that are rather stereotypical.
At the very beginning of the film, a No Parking sign is seen. These signs were written in Mandarin, Spanish, English, Arabic and Russian, and were especially made for the 1968 Olympic Games. The street depicted is the Eje Central Lazaro Cardenas, one of Mexico City's main boulevards, where the Palace of Fine Arts and the Bank of Mexico are located.