AS SOON AS he appeared on screen, stomach lurching, he uttered the lament.

"Oh no."

Jayson Tatum instinctively withered in his seat, shrinking lower as if that would somehow make him invisible.

It didn't. And as Kemba Walker, Khris Middleton and Donovan Mitchell turned their attention to Tatum with equal parts sympathy and amusement, he braced himself for what was coming next.

"I wish I could be you!" boomed USA Basketball coach Gregg Popovich, striding toward the young Boston Celtics forward in a film room in China. "Then I could stand in the corner with my 7-foot-long arms down by my side and not move while players go past me!"

Popovich halted in front of Tatum, imitating his slumped position as his teammates unsuccessfully stifled peals of laughter. Then, the coach smiled, tapped Tatum's shoulder reassuringly, and clicked the frame forward to unearth the next defensive truant.

"It kind of reminded me of college," Tatum says, when asked about the moment some five months later. "Coach Pop and Coach K have similar personalities in how they lead. Pop enjoys calling you out in front of everyone.

"Confrontation, you know? It really makes you think about rising to the challenge."

"It was awesome," says Marcus Smart, who was in the film room with the U.S. squad. "Pop brings out the intensity by doing that, makes you want to be great."

What Smart and the others were not privy to was what happened in the days that followed that film session. Tatum was hustling to pick up his training gear when he ran into Popovich in the hallway. He motioned for Tatum to take a seat on a bench next to the elevator, slung his arm around the 21-year old, and explained why he singled him out.

"Do you know how special you can be?" Popovich asked. "There are very few two-way players in our game, and you have the opportunity to be one of them. You could be like Kawhi [Leonard] and Paul George."

Tatum was so surprised he didn't answer at first, prompting Popovich to hastily add, "I hope I haven't offended you."

"Offended me?" Tatum said. "That's a compliment!"

EVERYBODY LOVES TO score. Racking up points means adoration from fans, coveted endorsements, anointment from sneaker companies. No wonder, then, true two-way players are elusive. It's taxing to guard the league's top players, and it can be a thankless job. Scoring is much more glamorous.

"Nobody cares about defense," Smart says. "Unfortunately, our game is all about entertainment. Nobody wants to see a low-scoring game. They want to see high-flying dunks, someone going off for 45 points. Nobody gets too excited about seeing a guy in a defensive position for 24 seconds actually stopping someone from scoring. That's not going to get you any commercials."

Yet Tatum says he has made a conscious effort to prioritize defense this season. And his coach has noticed. Tatum has all the tools to be an elite defender, says Brad Stevens, but a consistent effort on that end is the only way he will truly cement his status as a stopper. The fact that Popovich spent the summer hammering home that same message, Stevens believes, was a gift.

"Jayson's length is a major factor, and he uses it effectively," Stevens says. "Some guys have the length and aren't necessarily active with their hands. Jayson is, not only on the ball, but off the ball as well."

Tatum excels at defending pick-and-rolls, according to Second Spectrum tracking data. Boston allows 0.82 points per play when Tatum defends the ball handler on screens, the sixth-best mark in the league among players to face at least 300 picks. And he boosts the team's overall defense. The Celtics have posted a 101.8 defensive rating with Tatum on the court, and a 107.3 defensive rating when he sits.

It has become an increasingly common sight to watch the lanky Tatum jump passing lanes and pick off passes like a defensive back; opponents have a 15.9% turnover rate with Tatum on the floor and 14.0% when he's off the court. That 15.9% turnover rate would rank in the top three of all NBA teams. Tatum also is blocking shots at a career-high rate.

Both Celtics president of basketball operations Danny Ainge and Stevens caution that defensive metrics can be unreliable. "Sometimes, they are way off," Stevens says, "but Jayson's defense is much improved."

It hasn't always been so easy.

When Tatum arrived in Boston as a 19-year-old rookie, Ainge had warned him, "They'll be coming for you." The speed of the game startled Tatum, and he constantly found himself one step behind the team's defensive schemes. Ainge was right: The new kid might as well have had a blazing bull's-eye branded on his back, as many of the league's most gifted scorers feasted on his inexperience.

"The narrative of me coming out of college was that I wasn't a great defender, and I didn't put a lot of effort into it," Tatum explains. "Guys were going right at me."

During that rookie season, Smart pulled Tatum aside and said, if nothing else, stay engaged at all times on defense. "You don't want to be 'that guy,'" he cautioned. Tatum was only three games into his NBA career when he realized what Smart meant. The Celtics were playing the Sixers when fellow rookie Markelle Fultz burned him on a backdoor cut. Tatum used his length to recover and block the shot, but that didn't spare him from becoming 'that guy' in the film room, the teaching tool of how not to guard a back-door play.

"I felt like I saw that clip on film all season," Tatum says.

JAYSON TATUM STILL hasn't fully processed what went so horribly wrong for him and the Celtics last season. His confidence was soaring following a rookie campaign in which he, Jaylen Brown, Terry Rozier and Smart nearly upended LeBron James and the Cleveland Cavaliers the previous season in the Eastern Conference finals.

He had grand plans to be a reliable starter, a double-digit scorer, maybe even an All-Star. With James migrating to the West, he felt certain the Celtics would have a shot at advancing to the NBA Finals.

Instead, the team -- and Tatum -- stumbled. Their group lacked cohesiveness, and, as the season wore on, the defensive commitment necessary to become an elite team. Too often, Tatum was part of the problem, a half-hearted defensive participant.

"Last year for me, especially when things weren't going my way, unfortunately it translated to defense," Tatum admits. "I wasn't as engaged when I wasn't happy. That happened a lot last year. They were some very tough days, more often than not. It really wasn't any fun."

Tatum was hardly alone. As Stevens said, none of the pieces they assembled seemed to fit. But, Ainge stresses, that doesn't forgive a lack of effort.

"There's all sorts of excuses players can make for why they don't play as hard and as well as they want to," Ainge says. "Ultimately, it's up to each individual.

"I don't care what sort of role you have, whether you're happy, or you get as many shots as you want. The one thing I try to tell all our players is, you still have to play defense. You better compete at the highest level, or we'll find someone else who will."

It has been well-documented that Boston's lineup was fractured in 2018-19 -- with younger players who hoped to take a huge step forward in their development chafing at their reduced roles, while strong-minded veterans Kyrie Irving and Marcus Morris Sr. took it upon themselves to show the "kids" how it should be done. At times, their approach was heavy-handed, and Brown called out Irving for laying the team's troubles solely at the feet of the young guys. For Tatum, whose closest companions last season were Irving and Morris, those conflicts led to some uncomfortable moments.

"I did kinda feel like I was in the middle sometimes," Tatum says. "You read all these things on the internet. I felt like every day last year when I woke up and turned on the TV they were talking about us, our locker room and all the chemistry problems. It was tough. You see that, and then you have to come to practice.

"We all tried to stay in a good mood for the most part, but everybody was frustrated. Nobody played, either individually or teamwise, the way they wanted to. Everybody had goals they wanted to reach and nothing was going our way."

It was Irving and Morris, he says, who counseled him through those struggles. "I miss them both. They were my guys," he says. As former Duke players, Tatum and Irving established a bond that was further cemented when Irving invited Tatum to join him and Kevin Durant on a vacation to the Bahamas in the summer of 2018. As Irving emerged a lightning rod toward the end of last season's disappointing run, Tatum stood by him.

"Some of the guys may have [resented] my relationship with Kyrie, but nobody ever said anything to me," Tatum says. "Kyrie and I are genuinely friends, always will be, no matter what people think. If you're my friend, I'm always going to have your back. And so, I had Kyrie's back."

Tatum says he talks with Irving from time to time. His former teammate has been hampered by a shoulder impingement through the first half of the season with the Brooklyn Nets, but when Irving has been out there, Tatum has perched himself on his chaise lounge with his son Deuce in his lap to see what his friend does next.

"He's my favorite player to watch," Tatum says. "After playing with him my first two years, I saw what he was capable of. Now, when I'm sitting on my couch watching him, sometimes I know what he's going to do before he actually does it."

THE VIBE IN Boston's locker room this season has been decidedly more upbeat, and Tatum quickly acknowledges Walker as a unifying force. "Everyone knows Kemba is great," Tatum says, "and the rest of us have worked really hard to put last year behind us."

Walker says he was aware of last season's dysfunction but chose not to delve into it, approaching his new team as a clean slate. He has organized team dinners and group outings. He has stressed the need for their group to remain communicative.

"We have to be a tight-knit group, because we see each other more than anybody else," Walker says. "So, from where I stand, we had to find a way to enjoy each other."

After their resounding win over the Lakers on Jan. 20, Walker, Tatum, Smart and Brown lingered by their cars, savoring the win. Their conversation turned to the American television crime drama "Power," and the four teammates shared their respective theories on where the plot was headed. It was a gathering that would have seemed unlikely a year ago. While Tatum and Brown have never exchanged a harsh word, last season the future cornerstones were not nearly as collaborative as they are this season.

"They are two special players," Walker says, "and this is their team. I love watching them when they are in a groove together."

"I really like how my relationship with Jaylen has grown this year," Tatum says. "We've known each other a long time -- all the way back to high school. It's been really exciting to take that next step with him."

Ainge says more physical players like LeBron James still give Tatum trouble, but at only 21, Tatum is still growing, still getting stronger and has become infinitely smarter and more competitive. "He's become a really reliable defensive player," Ainge says.

Of course, there's always the lure of offensive glory, and Tatum will still succumb to it. Consider the end of a blowout win over the undermanned New Orleans Pelicans on Jan. 11, when Tatum was on the verge of scoring 40 points, which would have been a career high. As the third quarter wound down, Tatum jacked up a pair of wild 3-pointers, and Stevens quietly motioned him over to chat on the sideline.

"I told him, 'Make the next right play,'" Stevens says. "We were actually trying a new version of a defensive set with that group, and I wanted those two to three minutes to close out the quarter to be used in the right way."

Tatum got his 40, but he also limited Brandon Ingram to 16 points on 26.7% shooting from the field. Ingram had come into the TD Garden averaging 25.6 points, 6.2 rebounds, 5.5 assists and 43.5% from 3 over the previous 10 games.

Two days later, Stevens was sitting in his office with his teenage son, Brady, who was talking to him about how hard it is to play good defense. He ran some tape on defensive help, and, unsolicited, the coach's son remarked, "Man, Tatum's pretty good at this."

"I texted Jayson and told him, 'I knew you'd score 40,'" Stevens says, "'but I didn't know was you'd be a defensive example so soon in your career.'"