NEW YORK, NY.- Joe Locke nailed his first audition. He was 6 and up for a part in an amateur production of “The Wizard of Oz,” and he easily secured a callback. But when he was told he would have to return for another audition, the thought of trying out again reduced him to tears. He declined the callback. The role went to a different child. “It did traumatize me a bit,” Locke said.

“And,” he added, ducking his chin toward the neck of his cream-colored hoodie, “I was always a crier.”

Locke got older. He got bolder. He threw himself into school plays, Christmas pantomimes, community theater in his home on the Isle of Man. “I come from a very small place where individuality is less embraced,” he said. Onstage, playing pretend, he could express more.

He planned to apply to drama school. Then a family friend alerted him to an open call for a new TV show, “Heartstopper,” a drama about queer English teens adapted from Alice Oseman’s emotion-heavy comics.

At 17, after multiple callbacks, he beat out thousands of other young men to play 14-year-old Charlie Spring, the only openly gay student at an English preparatory school. “Heartstopper,” which became one of Netflix’s most watched shows when it debuted in 2022, was a showcase for Locke’s boyish, beetle-browed charm and uncanny ability to wear his heart on the sleeve of his rugby shirts.

Locke isn’t accustomed to giving himself compliments, but he offered one here: “I can be quite good at being vulnerable onscreen,” he said.

If Locke became a star playing one untried adolescent, he will most likely become even more famous for playing another, a character known as Teen, in “Agatha All Along.” A Marvel series spun off from “WandaVision,” it follows that show’s witchy villain, played by Kathryn Hahn, as she assembles a coven and tries to regain her powers. The first two episodes premiere Wednesday on Disney+. The third season of “Heartstopper” follows on Oct. 3.

“I’ll play as young as I can for as long as I can if that’s what people think I can play,” Locke said, his lips squeezed into a happy-sad half smile. “And no, I’m not mad about that.”

He was speaking from his London bedroom via a video call, his dark eyes blazing through the screen. He was two weeks shy of turning 21, and if he didn’t seem exactly relaxed — Zooming with a stranger is rarely a recipe for ease — he was confident, self-possessed, a grown-up, almost.

The audition process for “Agatha” was longer and more fraught than anything in his experience. There were self tapes, and then several online callbacks. While rehearsing “The Trials,” a climate crisis play at London’s Donmar Warehouse, he was asked to come in for an online chemistry read with Hahn, so he made a whiplash trip to Los Angeles. Walking into the audition room, he felt unbearably anxious. Then he looked at Hahn and the anxiety fled.

“It felt like everyone else in the room just went away,” he said.

Jac Schaeffer, the “Agatha” showrunner, remembers how Hahn responded to Locke. Hahn is an outsize performer, an extreme weather event onscreen. Though Locke is younger and slighter, he entirely held his own.

“When he came in, everything in the room changed,” Schaeffer said. “He was commanding. He changed Kathryn’s performance.” When the scenes were done, Locke and Hahn noticed that they were wearing the same pair of Vans sneakers, fashion kismet. There were more callbacks after that, but the part was his.

The series shot last year in Atlanta. Locke lived on his own for the first time, in a three-bedroom house, which could be scary. But the set was a joy. He liked to sit and listen to his co-stars playing the witches — Patti LuPone, Sasheer Zamata, Ali Ahn, Debra Jo Rupp, Aubrey Plaza — as they joked and cackled and sang. Having grown up with a single mother and several aunts, this felt like home.

“I remember, my first day, Patti laughing like, ‘Oh God, you’re stuck with all of us,’” Locke recalled. “And I was like, this is where I feel comfortable.”

Occasionally, he would ask the women for advice about certain career moves and they asked him questions, too. “We were like, How did this happen?” Hahn said. “His self-possession and his immediate success right out of the gate, we were just trying to get to the bottom of it.”

Some enigmas remain, especially those surrounding Teen. In the first episode, he presents as an ordinary adolescent with a serious eyeliner habit and a particular interest in witchcraft. (Teen is also gay, which fulfills a personal goal — Locke once told an interviewer that he would love to play a gay Marvel superhero.) Marvel swears its actors to secrecy, and Locke was careful about what he would and wouldn’t say.

“He is mysterious,” he said of Teen, which wasn’t exactly helpful. “He holds a lot of the secrets of the show.”

Locke put a lot of himself into Teen, just as he put a lot of himself into Charlie, and into Tobias, the character he recently played in “Sweeney Todd” on Broadway. “There’s something elemental about him,” Thomas Kail, who directed that production, said. “It’s hard at any age, but especially early in your career, to not be afraid to use yourself.”

Locke doesn’t have that fear, and he doesn’t know another way to create a character. Even in the middle of real-life emotion, he will often find himself thinking about how he can use it in his work.

“Sometimes when I cry, I’m like, Oh, this is great,” he said.

He doesn’t shy away from difficult material — colleagues say that in some ways, he prefers them. “He finds the trickier stuff, the harder stuff, the darker stuff much more enjoyable and much more fun,” said Kit Connor, his “Heartstopper” co-star.

The upcoming third season explores Charlie’s eating disorder. It also shows the characters, including Charlie, in sexual situations. Locke mostly embraced these more adult scenarios. Though he was nervous in anticipation of the sex scenes, the actual shoot felt easy. Some days it was almost too easy.

“I was so into the character at that point, it’s almost like am I doing enough?” he said.

Locke isn’t really Charlie. Or Teen, for that matter, or any of the other characters he might play. He’s more confident, more assertive, mordantly funny and not so tightly held. “If there’s an alternate-universe Charlie, louder and more annoying, that would be me,” he said.

He had plans for his 21st birthday: He wanted to be home, on the Isle of Man, with his family and a Marks & Spencer cake. He also has plans for his career. He’d love to do a low-budget indie, he said. Something dark, something different. But he isn’t in any hurry.

“I’m really into the idea of proving to myself and to the world that I’m actually an actor,” he said. “But I have so much time. As long as I don’t get hit by a car, I have time to explore.”

This article originally appeared in The New York Times.