The Season 2 opener finds Guillermo, still hoping to become a vampire, threatened by the arrival of Topher (guest star Haley Joel Osment). He’s a bro-ish familiar (vampire-in-training), a Lehigh graduate “proficient in Excel” who wears an ever-present Bluetooth-type headset and uses terms like “dope” and “dog.” He’s a passive-aggressive threat to Guillermo, who’s spent over 10 years as Nandor’s familiar and resents the intrusion.

“I think Guillermo is protective of Nandor because, after so many years of service to someone, you either learn to love your boss or you wouldn’t be there,” Guillen tells The Post. “Everyone has a breaking point; Guillermo would leave [Nandor] but something is keeping him there. Every year that goes by is a second to Nandor but Guillermo hasn’t lived his life to the fullest. It upsets him and maybe he can be selfish in that way, but he has to be a great familiar to Nandor in order to make him a vampire.

“He’s this hard-working big dreamer.”

Guillen, 29, has been with the FX mockumentary series since it premiered last March with Proksch, Novak and Natasia Demetriou and Matt Berry as bickering husband-and-wife vampires Nadja and Laszlo. Guillen says he won the role in a very unexpected way — while attending a wine-and-cheese party thrown by a friend.

“The next day I got a text from one of her friend’s saying, ‘I think you’re so funny. You should audition for [the role of] Guillermo,” Guillen says. “I wasn’t falling for this old Hollywood trick, but my friend told me the person who sent the text was [executive producer] Garrett Basch’s fiancee. My agent said I was too young for the role — the character was written as 20 years older — but I said, ‘Her fiance works for [“Shadows” executive producer] Scott Rudin. I think we can trust a stranger I met at wine-and-cheese night.’ ”

Guillen hoped to audition for series casting director Allison Jones, but she was in London, so he put his audition on tape. Shortly thereafter, he got a call from Taika Waititi, who co-created the series with Jemaine Clement, telling him he’d gotten the part and “to be on the set next week” (the show films in Toronto).

Guillen didn’t know any of the other casts members, but he and Novak “really hit it off” when they met for the first reading of the script, he says.

“I’m the luckiest boy in the world,” Guillen says. “Kayvon is an amazing actor and comedian with the biggest heart. It’s fun doing the table (script) reads with him because they’re so effortless. Usually for the table reads they seat us next to each other because we’re more-than-likely in the same scene. We don’t act out [the script] but Kayvon always puts on his Nandor voice, which is helpful for me. Normally, when I talk I’m very loud — but when I’m Guillermo that becomes very soft.

“I think what’s so great about Guillermo’s story is that he’s exploring different options, powers he never knew were in his possession,” he says. “It’s relevant to what we all go through — you think you want something so badly but the universe tells you, ‘Nope, look over there.’ Maybe there’s something in front of you and you consider an alternative to get to that final destination.”

Guillen says eagle-eyed viewers may notice slight changes to Guillermo’s wardrobe. “This year some of Guillermo’s sweater-vest color pallette has evolved,” he says. “Last year it was mostly browns and creams. We’ve gotten a little more adventurous this year; there are hints of turqouise. [Costume designer] Amande Neale found some sweaters and dyed some of them from their original bright-orange color.”

And he says his role on “What We Do in the Shadows” has won him a legion of fans.

“People doing Cosplay are getting [Guillermo’s] glasses and the sweater and the wooden stake,” he says. “They’re saying, ‘Whenever Comic Con comes back, we’re ready.’ “