The episodes were shot in order and the actors only received the scripts for each episode as they were shooting, so they also did not know what their character's fate would be. Álvaro Morte (The Professor) said the actors would message each other with excitement when they received a new script.
The song used several times in the series, "Bella Ciao," is a partisan hymn, adapted from an old Italian country folk song from the latter part of the 19th century. During WWII, it represented the Italian resistance against Benito Mussolini. It is used worldwide as an anti-fascist hymn of freedom and survival in hard times. This tune has been re-recorded by many famous singers and translated into many languages. Even nowadays it can be found in protests and resistance situations around the world.
This Spanish TV series was the most watched non-English show on Netflix since its release, Until Squid Game (2021) was released and became the most watched non-English show on Netflix.
Denver's unique laugh was written into the script even before Jaime Lorente was cast in the role. The script simply called for "trashy laughter" and each actor who auditioned interpreted that differently.
The character of "Tokio", from her haircut to her clothing style, was highly inspired by the character Mathilda (Natalie Portman) in the film Léon: The Professional (1994).