There’s been a lot of hype leading up to Fantastic Beasts: The Crimes of Grindelwald—this is set in the Harry Potter universe—but it’s especially loud around one mysterious new character: a woman, played by Claudia Kim, described on fan-site Pottermore as a “person cursed to become a beast.”

Intriguing, right? Naturally, this was enough to set the Twitter-sphere off with theories in the months leading up to the film. Then came a bombshell: Kim is Nagini, a woman who will eventually become the pet snake-slash-Horcrux of evil Lord Voldemort.

Naturally, fans had a lot of thoughts about this. And at the center of the swarm was Kim, an actor suddenly catapulted from a relative newcomer to the key player in a beloved series.

Luckily, Kim has experience with major franchises—and their intense fanbases. After starting her career as an actress and model in South Korea, her first major roles in the United States included supporting turns in Avengers: Age of Ultron and The Dark Tower. Even so, Nagini and Harry Potter is on another level. (Tony Stark wishes.) “It’s mind-blowing to be a part of these franchises,” she tells Glamour. “It’s not something I had imagined would happen to me, especially so quickly.”

The role was a personal breakthrough for Kim, too. In South Korea, she says she was mostly offered action or thriller roles. Eventually, she felt restless and wanted more for her career. “I was at this point in Korea when I thought, I want something different,” she says. “Not all the roles that I’ve gotten were stereotypical, but in Korea, especially for TV, it’s a bit limited for women in their twenties and thirties. There aren’t enough female roles.”

So, Kim made it happen for herself. She landed those aforementioned parts in Avengers and The Dark Tower, as well as a role in Netflix’s Marco Polo, before getting Fantastic Beasts. It was a gamble: Kim didn’t even know what, exactly, she was auditioning for until she got the part. And competition was fierce, despite all the secrecy. Kim stood out by tapping into Nagini’s sensitive side. “Actresses from all over the world read for the role, and Claudia’s read was the most emotional and affecting,” Fantastic Beasts director David Yates tells Glamour via email. “She was able to capture the character’s emotional core and found elements of both vulnerability as well as strength in what Jo [Rowling] had created on the page.”

“What drew me to the role was that she’s still very much a powerful, powerful being, despite her unfortunate circumstances and her vulnerabilities,” Kim says. In her hands, Nagini is more than Voldemort’s pet—she’s a sympathetic character dealing with real trauma: a blood curse that will eventually, and permanently, turn her into a snake.

Ezra Miller, who shares several scenes with Kim in the film, was blown away by their first read together. “With total respect to the other actor who was auditioning that day, I do remember that when Claudia left the room, we were still crying,” he says. “Consensus seemed pretty immediate on all fronts: Claudia is a superstar.”

Once Kim did find out where Nagini’s story led, that made the part all the more powerful to her. “In general, the way J.K. takes these marginalized characters and makes them understandable, there’s no ‘good’ or ‘bad’ or clear division,” she says.

“We always explored the balance between the vulnerabilities of the character who in this story is an outsider within the wizarding world, with this wonderful spirit and strength that Claudia could often find,” Yates adds.

It’s a teetering, trapped feeling that draws Nagini and Credence together, prompting them to set out on the run. (You’ll find out why in the film.) “They both have terminal conditions, [and] they are completely isolated,” Miller says. “All that being considered, they have somehow found this way to recognize each other and, to some degree, to trust one another. That brings them together on this journey as they’re both seeking themselves, seeking understanding and compassion and fortitude in their suffering.”

If Kim’s vulnerability makes the human half of her character believable, intense preparation was necessary for the other half, the serpent. She trained with a movement coach to incorporate subtle, snake-like motions into her scenes. And to bond ahead of their on-screen journey, Kim and Miller took an expedition to a solstice ritual—on-foot and through the woods. “It was fantastic because we kind of had to support each other on a journey…that was just the heart of our characters’ situation right there: People who are kind of getting to know each other really fast because there’s some sort of mutual understanding at play, who are sort of lost in the woods,” Miller says. Kim was down for the challenge. “Even that day, I remember us joking and being like, gosh, we’re doing such a great job at preparing for our character dynamic right now. We are genuinely lost in the woods looking for wizards.”

Then there were aspects of the role that were familiar to Kim and her background with action films. Fantastic Beasts required intensive stunt work; and according to her cast mates, she was the most active of the bunch. “It was the joke of the movie for me that every single scene, surprise surprise, Claudia in the skin-tight dress that doesn’t exist, in the 6-inch heels that are not made for walking or being human, and with her hair up, was inevitably doing something that the rest of us couldn’t physically do in gym clothes,” Miller says.

All stunts, special effects, and spell-casting aside, Kim says the most memorable parts of the role are the most emotional. Magic or muggle, everyone can find common ground with her character. “She wants to be accepted, and she wants to be free,” Kim says. In other words, “It’s fantasy, but it’s relatable.”

A version of this story originally appeared in the December issue of Glamour.