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.[8] Going though 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.[2] The producers also found that his appearance of a primary school teacher gave the character more credibility.[3] 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.[10] Morte, who does not wear glasses in real life, developed a tic of pushing back his glasses during casting; this was left in the show for the originality of the performance.