Michael Morse, regarded as one of the Nationals’ comic ringleaders and resident DJs, is a popular presence in the clubhouse. (Tom Lynn/ASSOCIATED PRESS)

It’s unclear exactly when Washington Nationals reliever Sean Burnett turned a white sheet into a makeshift toga and served his teammates on a flight. He did it earlier this season, likely on the team’s flight from Philadelphia to Atlanta in late May. He ambled up and down the aisle of the charter plane, passing out food and drinks to the team like a flight attendant for the entire trip wearing only the sheet and underpants.

“Every time he’d walk by, we’d yank on it,” first baseman Adam LaRoche said. “Eventually it got stretched out to be 48 inches in the waist and he couldn’t keep them up anymore.” By then, the players were consumed with laughter.

The Nationals carry the best record in baseball on the field, but they’re also a vibrant and jovial group of players off it. They smile and laugh in the dugout, the bullpen and behind the scenes — enlivened by winning. But few of the players have ever been in a pennant race, and the light-hearted tone set by veterans is as important now as ever.

“We’re delving into waters a lot of guys in this clubhouse have never been in before,” said Mark DeRosa, considered one of the team’s most vocal leaders. “And I think the last thing you want to do is not be loose and not play up to your capabilities.”

Winning makes it easier to relax. Players say coaches cut them more slack when they’re churning out victories. And so far, they’ve found a good balance. “It makes it easier to come to the field every day when you’re friends with your teammates,” said third baseman Ryan Zimmerman, who endured seven losing seasons in Washington until now.

If the Nationals were a comedy show, DeRosa would be the main act, many say. The 15-year major league veteran won a World Series ring with the San Francisco Giants in 2010 and has played in the postseason six times. And though DeRosa has played little this season, his impact behind closed doors is vital. He is part team dad and part team comedian.

Perhaps the most vital piece of equipment in the Nationals clubhouse is a suitcase-sized black speaker that sits in an empty locker between DeRosa and Michael Morse’s lockers. During a series in New York in late July, music aficionado Morse saw the speaker system with a microphone and iPod hookup, loved it and paid the clubhouse attendant about $500 to buy one.

DeRosa puts it to good use. With the delivery of a seasoned standup comedian and tinges of his New Jersey accent, DeRosa rags on anyone in the line of sight: in the clubhouse, plane and bus rides. He calls himself “a verbal assault machine.”

One quiet Sunday morning in August before a day game against the New York Mets, a reporter wanted to interview Ian Desmond. While singing intermittently over the Beatles’ “Let It Be,” DeRosa suggested that the reporter ask his questions over the microphone. On Kurt Suzuki’s first day in the clubhouse after arriving in an August trade, DeRosa read the new arrival’s Wikipedia page over the portable speaker for all to hear. Line by line.

“If you don’t have thick skin, you can’t come in this clubhouse,” DeRosa said. “Including staff.”

Everyone does their part to keep the team’s spirits high. In the bullpen, during games, relief pitchers talk and joke about anything from fantasy football to what pitch the opposing batter swung at. On flights, bus rides and in the clubhouse, Morse is the team DJ. He has an expansive music collection, anything from ’80s classics to Notorious B.I.G. to modern hits, and plays it for all to hear.

“He has so many things on his iPod,” rookie Bryce Harper said. “And he finds the funniest videos on YouTube.” Morse described plane rides as “a club in the air.”

Jordan Zimmermann is the team’s unexpected witty one-line jokester. Gio Gonzalez is incessantly bouncing around and talking with everyone at once. Morse is perhaps the second-most cited comedian of the clubhouse.

Some teammates point to LaRoche as a quiet prankster. LaRoche, a veteran who mixes humor with sage advice, sent Miami Marlins Manager Ozzie Guillen an autographed Harper bat smeared in pine tar after Guillen accused Harper of using too much pine tar during a game. “He’s the silent assassin,” Chad Tracy said of LaRoche.

Some even said a teammate, possibly LaRoche, slipped bananas and sunflower seeds in the equipment bag of then-teammate Xavier Nady before he went to Class A Potomac on his minor league rehab assignment. LaRoche wouldn’t admit to being the author of the prank.

“It’s classified,” he said with a broad grin across his face. “That’s why I wear my camo around the clubhouse, slide in and out, go hit somebody, get out and nobody ever knows I was there.”

Other popular pranks among the team are putting Hot Stuff ointment in clothes, dumping baby powder on someone in the bathroom stall or putting shaving cream in the bathroom towels. Some players were reluctant to reveal some of the funniest moments because they’re private, and those likely wouldn’t be suitable to be repeated here.

Sometimes, however, their fun leaks out. Last week, the veterans performed the embarrassing annual tradition of rookie hazing. The first-year players donned red skin-tight leotards on the train ride to New York, posing as members of the women’s gymnastics team with gold medals around their necks and a USA Olympics flag to carry. Gonzalez shared the moment with all by tweeting out photos.

With less than three weeks left in the regular season, the Nationals are in prime position to lock up their first playoff berth and the National League East division crown. Their talent has become readily apparent to their opponents. Their personality has, too.

“You know you’re having fun when guys from the other teams are saying, ‘Man, you guys are having fun.’ ” Morse said. “. . . When all of this so-called pressure and other stuff starts coming along, we just keep maintaining what we’re doing.”