BOSTON — It was an otherwise nondescript afternoon last summer at a small gymnasium in Southern California, with various N.B.A. players cycling through for off-season workouts.

But when Ryan Razooky, a 26-year-old basketball trainer, struck up a conversation with Boston Celtics guard Marcus Smart, he immediately sensed that something unusual was happening. As Smart began an extemporaneous monologue on the art of on-the-ball defense, Razooky gestured for his intern, Ryan Arevalo, to grab his hand-held video camera and record everything that Smart was sharing.

The result was an 18-minute video that has since been viewed on YouTube more than 540,000 times, a window into the mind-set of an elite defender that illustrates the hard work that goes into preventing high-octane scorers like Stephen Curry and Damian Lillard from doing what they want to do, which is put the ball in the basket from nearly every conceivable angle.

“It’s tough,” Smart said in a recent interview. “It’s tough to sit in a stance for 24 seconds and guard some of the best players in the league.”

It has been a trying season for the Celtics, who were 18-21 after blowing a 25-point lead against the Knicks on Thursday. But Smart, a 6-foot-3, 220-pound wrecking ball, again ranks among the league leaders in steals, averaging a career-best 1.9 per game. Finding players who can defend — and defend multiple positions — has seldom been more in demand as players of all shapes and sizes continue to expand their offensive skillsets.

Consider the evolution of the game since Smart entered the league in the 2014-15 season, when teams averaged 7.8 3-pointers per game while averaging 100 points, according to Basketball Reference. Last season, teams averaged 12.7 3-pointers and 112.1 points per game, notable increases that helped spur the league to amend a few rules before the start of this season that were aimed at reducing advantages for offensive players. No more lunging forward on shot attempts to create contact and draw fouls? Smart said he was all for it. But there are still nightly challenges.

“It is a thankless job,” Razooky said. “It’s like being an offensive lineman.”

There is something of a mutual admiration society for N.B.A. defenders. Smart, for example, spoke highly of the Bucks’ Jrue Holiday and the 76ers’ Matisse Thybulle, who has unique capabilities as a perimeter defender.

“He’s blocking 3-point shots, which is something I’m not doing,” Smart said.

Yet Smart has long been known for his own defensive prowess. At Marcus High School, which is outside Dallas, his coach, Kenny Boren, recognized Smart’s uncanny instincts for the ball and had him guard the opposing team’s worst player so that he could roam the court as a free safety, picking off passes and drawing charges as a help defender. It was an unorthodox philosophy. The team won back-to-back state championships.

“He had a free pass on defense to do what he wanted,” said Boren, who now coaches at Boswell High School in Fort Worth, and helps run Smart’s youth camps. “But you could trust him to make the right play.”

Razooky had never met Smart until that day last summer when Razooky approached him after his workout.

“I was just joking around with some of the coaches who were helping us,” Smart said, “and they were asking me about defense and how I’m able to guard certain guys and whether I had any tips.”

Smart did, in fact, have tips. It was like asking Neil deGrasse Tyson about the solar system. And for 40 minutes, Smart was a fire hose of insight, holding court on topics dear to him.

He demonstrated how he flicks for steals when an opposing player’s dribble is on the descent. He emphasized the importance of lateral quickness, citing the quick hips he developed as a football cornerback growing up in Texas. He offered pointers on fighting through screens (grab the screener’s ankle and pull your way through), and on how to ensure that opponents become less inclined to set them in the first place (make hard contact).

“I’m going to hit you one good time,” Smart said in the video. “Referee is going to call it or he’s not. But I bet you won’t set no screens.”

Not that Smart has been immune from the physical demands of his job: In 2019, he tore a lower abdominal muscle after colliding with Nikola Vucevic of the Orlando Magic.

“Your hips take a beating,” Smart said in an interview. He used to wear padding when he was younger, “but now, they get too heavy with sweat and weigh me down.”

He also highlighted the challenge of defending step-back jumpers, the type of offensive move that the Nets’ James Harden and the Mavericks’ Luka Doncic have worked to perfect. Smart mentioned how he had studied the martial art Muay Thai to improve his balance and his core strength, and how he stays on the balls of his feet “like a boxer” to avoid getting swayed by crossover dribbles.

Smart said that it took him about a full year to develop the necessary footwork to defend those step-back jumpers.

“It sounds weird, but as a defender, you have to go out there and practice the things you’re trying to do, and I constantly do that,” he said. “I tell my friends at workouts, ‘All right, here’s what I want you to do with the ball, and I want you to try to beat me because I’ve got something I want to try.’”

The twist, Smart said, is that he was somehow oblivious to being filmed by Arevalo, the intern. (“You could see his passion,” Razooky said.) Smart generally does not want his trade secrets to go public. He has never understood why so many N.B.A. players share their off-season workouts on social media, because he knows opposing players — like himself — are mining those clips for information.

“If you want to show me what you’re working on so I can prepare for it, that’s fine with me,” Smart said.

But at the same time, he really enjoys talking about defense. And he ultimately agreed to let Razooky post the video.

“When you’re good at what you do, it doesn’t matter who knows,” Smart said. “They still have to go out there and compete.”

Razooky wound up editing the full 40 minutes down to about 18. A future director’s cut could include some particularly high-level instruction, like Smart waxing poetic on a spin move that he sometimes uses on defense.

“It’s something he can clearly do,” Razooky said. “But I’m not sure it’s something we necessarily want to teach other players.”

Smart was up to his familiar ball-hawking tricks against the San Antonio Spurs on Wednesday. In the first quarter, that meant deflecting an entry pass that was intended for Jock Landale, a 6-foot-11 center whom Smart was fronting in the paint. Smart later came away with two more steals in the game’s final minutes — first, when he stripped the ball after the Spurs grabbed a defensive rebound, and then when he stole an entry pass. Typical of their season, the Celtics still lost by 2.

A couple of weeks before Razooky uploaded the video in August, he shared a 15-second preview on social media. It caught the attention of Davion Mitchell, a defense-minded point guard who had been drafted by the Sacramento Kings. Mitchell, Razooky said, was eager to learn more before the start of his first N.B.A. season: Could he have a copy of the whole thing?

Now, everyone has access. Smart knows the audience has presumably included fans, coaches, young players and even some of the players he has to defend.

“I’m sure a lot of those guys are watching that video and trying to figure out, ‘OK, when he does that, how can I create something else to get around it?’” he said. “And while they’re thinking about all that stuff, I’m thinking about what I can tweak to counter their counter. It’s a chess match.”