Development
Early in development, the Professor was intended to be the narrator of the series. Writer Javier Gómez Santander compared the writing process to the Professor's way of thinking, "going around, writing down options, consulting engineers whom you cannot tell why you ask them that", but noted that fiction allowed the police to be written as dumber or more inept when necessary.
The Professor was designed as a charismatic yet shy villain who could convince the robbers to follow him and make the audience sympathetic to the robbers' resistance against the powerful banks. However, developing the Professor's role proved difficult, as the character did not follow typal conventions and the producers were uncertain about his degree of brilliance. While the producers found his "Salva" alter ego early on, they were originally looking for a 50-year-old Harvard professor type with the looks of Spanish actor José Coronado. The role was proposed to Javier Gutiérrez, who declined because he was already committed to starring in the film Campeones.
Meanwhile, the casting directors advocated for Álvaro Morte, whom they knew from their collaboration on the long-running Spanish soap opera El secreto de Puente Viejo, even though his prime time television experience was limited at that point. Going through the full casting process and approaching the role through external analysis rather than personal experience, Morte described the Professor as "a tremendous box of surprises" that "end up shaping this character because he never ceases to generate uncertainty", making it unclear for the audience if the character is good or bad. The producers also likened his appearance to that of a primary school teacher and believed this gave the character more credibility. The Professor's glasses were initially red and more prominent, but in the end the producers preferred a more discreet option and in a darker tone. The Professor's fascination with origami was proposed by Morte.
Creator Álex Pina saw Raquel's story as a formerly abused wife falling in love with the Professor "as very powerful, very romantic". That they come from two opposite sides was "intended to enhance the genre" and something that the producers "wanted to exploit". Le Monde said, "The game between Professor and Raquel, the inspector in charge of the negotiations, is brilliant. A step of two with fluidity and anticipation. To ignore one's enemy is to condemn oneself to defeat. It therefore requires to enter into his intimacy. Morte said the Professor would not necessarily prioritize love over other risks in part 3, saying that if 'he didn't have that other dark part, it wouldn't work either'. The Professor's trick that hooks us so much is that we never know what goes through his mind, what he is thinking. I try to give that ambiguity." The character's anger about Berlin's death spawned the character's motivation for part 3, and was an idea Morte gave the producers. In September 2019, Morte considered the Professor a "very interesting character" who had more of a story left to tell. Morte said about part 4, "the teacher makes a descent into hell."
