Louis McCartney knew to anticipate the fandom.

“The best thing about doing a ‘Stranger Things’ prequel theater show is that the fans, the nerds, are already there with arms open, waiting for you to deliver,” says McCartney, who’s making his Broadway debut as villain Henry Creel in “Stranger Things: The First Shadow.”

The popularity of the Netflix series — and impending finale — adds some pressure to the production, which McCartney describes as a more finessed version of the West End staging. McCartney, who originated the role in London, promises a Broadway show that’s harder, faster, better — and packed with more high-tech onstage illusions including levitation and elaborate set pieces that arrive from all directions. “We are really smacking people in the face again and saying, hey, you know, this is ‘Stranger Things,’ we’re here to play,” he says. “We’re doing it right, and we’re not missing a beat.”

It’s a couple days before the show’s technical lock and one week before opening, and the 21-year-old Irish actor is in his dressing room at the Marquis Theater, appearing calm before the storm. McCartney has already spent many months inhabiting the physically demanding role, which requires him to convulse onstage several times each show as the character’s body is overtaken by the “Mind Flayer,” a sinister supernatural force.

The play, set in 1959, serves as a character origin story and as a prequel for the TV show. But while crossover elements from the series — the recognizable theme song, several beloved characters, the “next episode” play bar — are met with cheers from theater audiences, the play also functions as a stand-alone piece. The production opens with a depiction of the Philadelphia Experiment, an alleged paranormal incident in which a WWII naval ship teleported — with lingering consequences for the Creel family, who relocate to Hawkins, Ind. Henry Creel draws ire as the strange new kid at his high school, and his arrival kicks off disturbing events throughout the community.

“I feel like every single time I talk about him, it changes. He is this labyrinth, this enigma, because he’s so contradictory,” says McCartney of his character. “He is a good boy; he wants to do the right thing. And he’s plagued by these visions of wrath and desire. And that’s this beast inside of him, the Mind Flayer.”

McCartney credits his TV counterpart Jamie Campbell Bower, who portrays an older version of Creel, as an inspiration for his portrayal. “You wanna be friends with him, even though he is a psychopath. And that’s the thing — he can be so charming and so disarming, and he can be really nice. We’re exploring that more in this play, because Henry Creel is a character who’s defined by his actions,” McCartney says. “ And in this play, every single choice he makes is him choosing his own fate. He’s always dancing between humanity on his right side, and the Mind Flayer on his left side.”

For McCartney, his own choice was between fencing and acting. As a teenager, his father, a screenwriter, urged him to try out acting — at the expense of his fencing class, which fell on the same night. It turned out to be the right call. The father-son pair started to film short monologues for YouTube, which led to McCartney booking his first agent and a role in the soapy Irish crime drama series “Hope Street,” which lasted for several years. A lead role in indie surfing movie “Silent Roar” followed, and in late 2023, McCartney made his theater debut with “Stranger Things.”

McCartney recently watched a documentary about the production’s early days, and was struck by how much he’s grown since it was filmed. “ I remember I messed up really badly the first week I ever did this production in London,” he says. “To be on stage and experience that fall was really eye-opening for me in a brutal kind of way,” he adds. “It opened my eyes to what theater is and how hard it is, and what it takes.”

So far, the work has paid off: a few hours before the show’s Broadway opening, McCartney received a Drama League Award nomination for distinguished performance, and last year he won the award for Most Promising Newcomer at the Critics Circle Theatre Awards in London. 

“Everyone [working on the production] are always on their A-game, and it’s been really nice to step into that again and not have that fear of it going wrong,” he says. “I’m super confident about it this year, and I’m so happy to be here and to be working on this again,” he adds. “It’s a dream come true.”