The actor Jamie Hector grew up in South Brooklyn, watching karate movies on Saturday mornings. He began studying martial arts as a teenager. “I wanted to throw a ninja star,” he said. “I wanted to fight like Bruce Lee.”

Three decades later, at a dojo in the Crown Heights section of Brooklyn, he squatted on the floor, an elbow arced above his head, a wrist bent back at an alarming angle. He was being marched around a golden, plant-filled room by his longtime sensei, Soke Haisan Kaleak.

“How am I manipulating him like a marionette puppet?” Mr. Kaleak said. Pretty easily.

At 46, Mr. Hector remains baby faced, with large eyes and a broad forehead that narrows to a tidy chin. He broke out 15 years ago as Marlo Stanfield, a stone-cold, sweatband-repping drug dealer in the third season of “The Wire.” With ruthless efficiency and a dreamlike stillness, Stanfield took over corner after corner. A lot of that efficiency came from Mr. Hector’s years of martial arts training, which taught him control, stamina, economy of motion and that absolute, unruffled presence.

“I consider it a spiritual discipline,” he said, his voice graveled like a pebbled beach.

He still practices whenever he can. And on this free afternoon a few days before the premiere of his new show, “We Own This City,” a six-episode HBO drama from the creators of “The Wire,” he had downed a peanut butter protein shake, then bounded up the stairs to Urban Asanas, a dojo that doubles as a yoga studio, so that Mr. Kaleak could nearly snap his wrist.

“I took you down,” Mr. Kaleak said cheerfully. “I could have done more.”

A spring wind chilled the streets below, but in this second-floor room, with houseplants tumbling down from hanging baskets and stretching up from clay pots, several space heaters hummed. Mr. Hector quickly broke into a sweat under his black gi, belted in brown.

Mr. Kaleak began the afternoon with a lesson on the two forms of blocking: interception and evasion. (His method, which he calls martial science, relies on a fusion of karate and jujitsu.) “You’re going to put something in the way or you’re going to move out of the way,” he said.

“Oss!” Mr. Hector and his dojo brothers shouted, a jujitsu term conveying that a concept has been heard and understood.

Mr. Kaleak then taught them acronyms for how to respond when confronted by an attacker: SMAC (shock, move, attack, control) and THE (throat, hands, eyes). Mr. Hector listened — head high, shoulders back, gut tensed, feet planted firmly on the floor.

After a few minutes of warm-up in which the students kicked at blue practice pads and punched the air, Mr. Kaleak announced a drill that he had named “in-country,” after his time in Vietnam.

“If you were ‘in-country,’ you would be in the thick of it,” he said. “So don’t be afraid of pressure. Take yourself to a point of pressure, whenever you can.”

In the drill, Mr. Hector would punch the practice pads with each arm, then evade strikes from each side, punch again, then use each elbow to block. He would run it three times.

“Drills make skills,” Mr. Kaleak said. “Your job is to make it muscle memory.”

Mr. Hector ran it for the first time, fists thwacking the pads, making a few errors.

“Always learn from your mistakes,” Mr. Kaleak counseled.

Mr. Hector ran it again with Black Hawk, a fellow student, holding the pads. His form was elegant, compressed.

“That’s the drill, that’s the pressure,” Mr. Kaleak said encouragingly. “Stand your ground.” Mr. Hector did, executing the drill a third time and a fourth until his blocks became more fluid. Would he feel it the next day? He might. This is what comes of training in your 40s.

“Now every hit matters, every strike matters,” Mr. Hector said.

From “The Wire” and on, Mr. Hector has made his acting matter, too. After playing a criminal, he switched sides and portrayed a detective on seven seasons of the Amazon Prime series “Bosch.”

In “We Own This City,” based on an investigation into Baltimore’s famously crooked Gun Trace Task Force, Mr. Hector plays Sean Suiter, a homicide detective and father of five. In contrast to the corruption that surrounds him, Suiter does his job with unshowy integrity.

“His north star is doing good,” Mr. Hector said of the character.

It’s a star he tries to follow in his own life. In 2007, inspired by the fourth season of “The Wire” that tracks a crisis in public education, he founded Moving Mountains, a nonprofit that offers free dance, voice and drama classes after school and on weekends to Brooklyn youth. (Moving Mountains moved online during the pandemic, but Mr. Hector is now searching for a new space.)

“The door is wide open,” he said. “That’s how we are able to serve so many kids and transition lives.”

Mr. Hector still considers himself a student. After the drill, Mr. Kaleak congratulated him on his persistence and his focus. “Good to see you, man,” he said, clapping him on the arm. “I want you to get that black belt.”

Mr. Hector didn’t intercept the challenge. He didn’t evade. “That’s a must,” he said. “Oh, yeah. Absolutely. We’re going all the way.”