[3] Imanol Arias was initially chosen for the alien's role, and the actor liked the idea, but it was eventually decided that he wasn't suitable for the character.

The producers successively tried to sign Burt Lancaster, Charlton Heston, Kirk Douglas and Robert Mitchum.

An agreement with Vincent Price fell through due to surgery, and his agent offered them Klaus Kinski instead.

According to Fernando Colomo, having to wait on a horse for a while angered him for the rest of the entire production and he was disrespectful to everyone except for Miguel Bosé and the Roma people who took care of the animals used in the film.

Colomo was left with a 50 million pesetas debt that he covered with the gains from his next film, La vida alegre.