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.
All of the original Cronos devices created for this film were stolen when production was completed. They were never recovered, so the Cronos devices that Guillermo del Toro owns are replicas from the original molds which Art Designer Maria Figueroa still had.
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 about having Mexican characters that are rather stereotypical.
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 have been good friends ever since, working together frequently.
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.
Guillermo del Toro: In the beginning of the film, walking a dog in front of Jesús Gris house with his real-life wife.