He has turned, often, to the show’s star, Victoria Clark, for guidance. She’s a 63-year-old Tony winner (for “The Light in the Piazza”), and he’s a 19-year-old novice, and in the show they both play high school students who are about 16 years old. “It was incredibly intimidating,” Cooley said. “I thought, ‘Oh God, she’s so experienced. She’ll probably be irritated by my amateurness.”

Instead, Clark has become some combination of mentor and friend; before each show, the two share a quick hug at the skating rink counter where his character works. “Of course I’ve felt very maternal toward him, but I’ve tried to put that aside, and I’m also aware, with my 40-plus years on Justin, that there could be some kind of power dynamic, but I’m super-conscious about that,” Clark said. “It wouldn’t make any sense to cast someone who was more experienced, because it would seem false, and with Justin there is a truth and beauty to his performance that I have to match. In many ways, he’s been my teacher.”

The show’s director, Jessica Stone, was originally hesitant about casting Cooley, because he was so green, but she said he has been an eager student, at one point pulling her aside to ask the questions he was nervous about asking in front of everyone. And she said the Clark-Cooley dynamic has paid off. “His youth and sweetness and openness, and his lack of the life that weathers us all, washes over Vicki,” she said, “and Vicki’s years of skill and work and experience wash over Justin.”

He’s had a lot to learn, and not just about easing up on the burgers and steaming his vocal cords. “I had to learn to feed myself,” he said. “How to not stay up late and play video games. It definitely took a step up in maturity for me to be working in the professional world. I’m still figuring out a lot of things.”

Tesori remembers a day when she ran into Cooley on his way to Trader Joe’s and he blurted out “Hey, how do you cook?”