From a long article by Kathryn VanArendonk for Vulture:
While Clement and Waititi looked for the right person to play Nandor, the vampire to Guillermo’s familiar, casting agents back in L.A. spearheaded the search for Guillermo himself. (“Most of the people they auditioned were white guys. It annoyed me so much,” Clement says.)
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“I don’t want anyone to imagine that we had the character super well-formed in our heads, and Harvey was the one who checked the boxes,” showrunner Paul Simms says. “We had some idea that he works for Nandor and he wants to be a vampire. It was more Harvey coming in with an original take that I don’t think we could’ve even described in words.”
For Guillén, who stumbled into an audition opportunity after learning about the role at a party barely 24 hours earlier, reading the script and thinking about the character held several important hints about who Guillermo should be. The first was simple: It may seem superficial, but for Guillén, the name Guillermo was key. “I was thinking, Do I know any Guillermos? And I thought, Oh, Guillermo del Toro. How cool he is, the side of him that’s so artistic and loves that world of darkness and goth. I thought, What a great name for someone who lives in that world.”
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Guillermo loves a collared shirt and a sweater: Simms describes his trademark look as the clothing of someone who “has a sense of who he is but also wants to blend in and look anonymous by choice. Someone who knows not to dress too nicely because he’s surrounded by vampires who are very vain.” In the pilot, as Guillermo celebrates his tenth anniversary as Nandor’s familiar and hopes he’ll get turned into a vampire, the collars on his shirt are sharp and very visible. “It was meant to represent fangs,” says Guillén, crediting the idea to Afiley. Once Nandor rebuffs Guillermo, the pointed shirt collars disappear beneath the line of his sweaters.
