Harry Shum Jr. Says He's 'Ecstatic' About His Superhero Debut: 'How Did I Even Get Here?'

Harry Shum Jr.'s superhero dreams are coming true.

In his latest film Legion of Super-Heroes, the actor, 40, takes on the mantle as the latest Brainiac, the notorious DC villain with 12th level intelligence, only this time he's helping the good guys. He stars opposite Supergirl, Superman's teenage cousin, and together they overcome their mutual animosity to attempt to defeat the Dark Circle, a group with a nefarious plot.

For Shum, joining the DC universe was a surreal experience.

"The influence that [DC] had on us as a kid is definitely making this even more exciting," Shum tells PEOPLE. "I was watching it the other day, the film, and I was like, 'What the hell, man? How did I even get here? How did I get to be able to voice these characters and to be part of this huge production?' So my 8-year-old self would be ecstatic right now, which I am as well."

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Shum stars opposite Zombies' Meg Donnelly (Supergirl) and his former Glee costar Darren Criss (Superman). However, his reunion with the fellow Glee alum was less climactic than Shum would have hoped.

"I filmed this during the pandemic, so the only way we got to really share our excitement was through text message, so having that reunion on screen yet off screen," Shum says.

Shum recorded the entirety of the film in the basement of his home. The Warner Bros. team had to weave together all the separately recorded voiceovers to make the scenes feel as though the actors were recording in the room together.

"It was definitely a challenge," Shum says. "I have all the equipment in my house, and it was like 105 degrees and trying to breathe within this small box I have, and in my head I was trying to read the lines and read it in a way that feels like a conversation."

He adds, "The crew really did a wonderful job just putting all these talents together and not being in the same room and to make it feel like we are. It's really a testament to the team and the animators to be able to make that feel so real and string it all together."

Shum is also starring on the latest season of Grey's Anatomy, where he plays new surgical intern Benson "Blue" Kwan. Shum says he was welcomed with open arms by the veteran cast, including Ellen Pompeo, who will exit after the show's 19th season.

"I think Chandra Wilson and James Pickens and Ellen Pompeo, they've seen it all, especially on the show. But one thing that was very universal within what they were trying to tell us is that, 'You're here because of you,' " the actor says. "You're here because of what you bring to the table. Always keep bringing that to the table. Don't let anything hinder that.'"

Legion of Super-Heroes will be released on Feb. 7 on Blu-ray and digital.