Mariska Hargitay, the longtime star of “Law & Order: Special Victims Unit,” recently stepped onto the stage of the Hudson Theatre, where she would soon make her Broadway début. “It’s just so magical,” she said, under an array of hanging light bulbs. Her intense “S.V.U.” character, Olivia Benson, investigates sexual-assault crimes; Hargitay, who is more lighthearted than Benson, had a snazzy new haircut and wore jeans, a boxy pink blouse, and lilac stiletto heels. This week begins her run in “Every Brilliant Thing,” the interactive one-actor London import. She is replacing Daniel Radcliffe; the stage was strewn with confetti and handwritten notes from his show the night before. “You know what’s so funny?” Hargitay said. “I saw the show twice, but I haven’t been here since then. I’ve been rehearsing in a normal room. So it just hit me. Yesterday they told me I have to look up.” She looked up: three tiers of Beaux-Arts splendor, opera boxes, Tiffany tile work, and nine hundred and seventy gold velvet seats. “I mean, look at this,” she said. “This is nuts.”

Hargitay, sixty-two and the highest-paid, longest-tenured actor on prime-time television, has played Benson since 1999. (Her fan base includes wearers of “HOT FOR HARGITAY” T-shirts and the Knicks hero Jalen Brunson, who hugs her, courtside, after home games.) She has produced and directed; she loves Broadway and has seen “Hamilton” twenty-seven times. Yet the new role is daunting: she hasn’t done theatre since high school. Her Playbill bio refers to her “side hustle for the last twenty-seven years” and thanks a nun who encouraged her to act in eleventh grade. “I certainly didn’t have a lot of stage credits,” she said.

“Every Brilliant Thing,” by Duncan Macmillan with Jonny Donahoe, centers on a narrator recalling growing up, a suicidal mother, and efforts to remind the mother about the innumerable “brilliant things”—ice cream, water fights, roller coasters—that make life worth living. (The handwritten notes suggested other brilliant things; one, near Hargitay’s feet, said “EVOLVING NICKNAMES.”) She said she’d felt immediately drawn to the play, and had a “beautiful exchange of energy” with Radcliffe after being dazzled by his performance. “What a light of a human,” she said. She’d told him, “I just want you to know that I’m the real Harry Potter.” She’d pulled her bangs aside and said, “See this?” Like Harry, her forehead is marked with a lightning-bolt scar. “He was, like—” She imitated Radcliffe’s stunned reaction. “We had this very deep connection—some kind of weird passing of the baton.”

The scar is essential to Hargitay’s origin story. As she details in the 2025 documentary “My Mom Jayne,” which she directed, she got it at age three, in a car accident that killed her mother, the actress Jayne Mansfield. Hargitay grew up with grief, as well as with the legacy of a famous movie-star mother she couldn’t remember. “I understand trauma,” she said. “Whether it’s your mother dying in a car accident, or being sexually assaulted—trauma is trauma.” Audiences relate to struggle, she added: “The only way out is through.” Vulnerability is strength; so is crying. “My tears aren’t weak,” she said. “Baby, I’m owning all of it. If anything, you should be scared.” She beamed.

Mansfield’s Broadway début had been at the theatre next door, the Belasco, in “Will Success Spoil Rock Hunter?” in 1955. “As I’m beginning a new chapter, it just can’t be an accident,” Hargitay said. She got this new part unconventionally. “They heard me say on ‘Good Hang with Amy Poehler’ that I wanted to do theatre,” she said. “I got the offer—magnificent. I called Amy immediately and said, ‘Hey, baby, thank you.’ ” On the podcast, they had “let it rip”: “I always thought I’d be a comedian, and Amy thought that she wanted to do drama. We laughed about that.”

“Before I got ‘S.V.U.,’ I had a development deal for a dramedy, à la ‘Ally McBeal,’ ” Hargitay went on. She’d ignored the prophecy of a psychic who told her that she’d move to New York and become famous for her serious face. Shortly afterward, she got “S.V.U.”—“so progressive, so groundbreaking”—and abandoned the development deal. Many viewers have confided their own sexual-assault experiences to her; she started a charity, the Joyful Heart Foundation, to support survivors. “I felt the same way about this play as I did about ‘S.V.U.,’ ” she said. “It felt like this gift—everything that matters to me. These themes are so brutal, and yet there is light.” She stood up and performed the first scene, as warm and exuberant as her regular conversation. Later, she cited some of her own “brilliant things” (Ping-Pong, mermaids, coziness, Linda Ronstadt, dragonflies). “Ironically, I’m obsessed with confetti,” she said. “If you look back, there’s photos of me like this.” She picked up a handful of confetti and threw it over her head, enraptured. “I do this all the time.” ♦