Accidentally-Correct Writing: The English trailer claims the Antichrist will be born in 2000, because it is exactly 2000 years after Jesus's birth. The Antichrist is born in 1995 in the movie. But historians believe Jesus actually was born between 7 and 4 BC, so it is possible 1995 was the 2000th year anniversary already.
Though Álex de la Iglesia knew Santiago Segura beforehand, he was actually suggested by producer Andrés Vicente Gómez after they had trouble casting the role. This is a rare happy instance of the trope, as many fans list Segura's performance as one of the best elements in the movie.
Pepe Sancho (Don Pablo in Cuéntame Cómo Pasó) was booked to play Cavan but he was replaced with Armando de Razza when the film became an Italian co-production. De la Iglesia had not the courage to tell Sancho that this was an imposition of the new producers, and Sancho never forgave him.
I Am Not Spock: Segura had to tell interviewers for years afterward that he actually doesn't like Heavy Metal.
Inspiration for the Work: As typical of Álex de la Iglesia, the plot spun from a scene he randomly thought. In this case, a frail looking, good natured man doing horrible things.
Never Work with Children or Animals: The black goat would attack the actors everytime they tried to record the scene in Cavan's apartment, so its appearance in the film was recorded with no humans present.
Sleeper Hit: The movie made over four million euros at the box office at a time when Spanish films averaged less than one, and basically convinced a new generation of Spaniards that there were Spanish films worth watching.
Throw It In!: Segura genuinely laughing at De Razza's fear of heights while filming the "Schweppes" sign scene led to José María receiving his uncontrollable laugh.
De la Iglesia first turned the project to Pedro Almodóvar, who had produced Mutant Action, but was turned down (at least according to legend, because the Raised Catholic Almodóvar was troubled with the idea of producing a movie featuring Satan, despite being openly queer and critical of the Catholic Church in his work). Another denegation by the Spanish Ministry of Culture put the film in Development Hell for two years, until Andrés Vicente Gómez took up the project on the condition that the budget would be limited to 300 million pesetas (Mutant Action had cost 400).
For obvious security reasons, the "Schweppes" sign was recreated in a soundstage and the street inserted in the background with bluescreen. It still was too high for Armando de Razza, who would complain about vertigo and force the crew to do shorter takes. Segura mocked De Razza while shooting, but later injured himself for real in the scene where Satan drops him from the Gate of Europe.
Most shooting took place at night around Christmas 1994 to make use of in-place Christmas decorations, with temperatures between 8º and -6ºC. Notice that none of the main characters ever wears a coat. De Razza had just a silk shirt on most of the time; Segura had his arms bare almost all the time, as makeup artists were reluctant to cover him between takes because cloth could mess up his fake tattoos.
The crew got permission to shoot at Callao Square for one night, but a passing local cop was skeptical and stopped the shot for four hours while he checked everything was correct. The crew had no caravans to stay in during this time, as it was in the middle of Madrid's shopping district.
Claimed Satanist groups made death threats to Álex de la Iglesia while filming.
Underage Casting: Pololo, who played José María's grandfather, was only six years older than Terele Pávez, who played his mother.
The first version of the movie was set in Sestao, in the Basque Country, was a straight Horror without comedy, a la The Omen, and contained long narrations of Ángel's thoughts as he silently reacted to events around him. This version was considered unfilmable, leading to the split of the main character between Ángel and José María, who would act as The Watson to his Sherlock.
Javier Bardem and Gabino Diego were considered to play José María.
Pepe Sancho almost played Professor Cavan.
The confrontation at the Gate of Europe was supposed to end with Ángel discovering that the baby (still alive) had a "666" written on its head and shooting it himself. This scene was filmed, but was considered so bleak that it was changed to the one in the film.
Write Who You Know: Álex de la Iglesia based Ángel on Jesús Igal Alfaro (1920-1986), his Ancient History university teacher who was also a Jesuit priest.
Other Trivia
Despite being in place since 1972, the movie is credited with turning the "Schweppes" neon sign atop the Carrión building into a cultural icon and being the main reason it was expressly excluded when neon sign regulations were tightened in Madrid in 2009.
The statue seen in the final shot is the Fountain of the Fallen Angel in Retiro Park, reputed to be the only public monument to Satan in the world and to sit exactly at 666 meters above sea level.
There were rumors of an American Remake in the works in the late 90s, when Álex de la Iglesia tried to get in the American market with the failed Perdita Durango and was on the run to direct Alien: Resurrection. However the claim that it would have been directed by De la Iglesia himself and starred Paul Giamatti as Ángel, Jack Black as José María, and Raoul Bova as Cavan was a 2011 Fools Day hoax in a cinema website (for an alleged 2012 release).
Became more Hilarious in Hindsight when Giamatti was surprisingly cast as a Professor Cavan-like character in De la Iglesia's 30 Coins - which is far from the only similarity between this production and The Day of the Beast.
