Joe Jonas Credit: Richard A. Photography

When Joe Jonas found out that he’d be stripping down to nothing but underwear with Charlotte McKinney for their Guess campaign, only one thought came to his mind: "I gotta stop eating this piece of cake right now.” To prep for the supersexy photos, the DNCE frontman, 27, picked up a rigorous three-and-a-half-month diet and exercise routine — but not without indulging a bit first. "Once I said yes, we were like, ‘It's the last meal,' and we just went crazy,” Jonas told Us Weekly exclusively at the Thursday, March 2, launch of Guess’ new men’s underwear line, Hero, at Macy's in NYC. "I had a lot of pasta and a lot of beer, because I didn’t have any beer during the training process.” Of the big night out, he joked: "I think I had one too many that night!”

Luckily, Jonas didn’t have to work out on his own. “Nick is always in the gym,” he tells Us of his younger brother. “He lifts heavy weight and I never really did that.” Joe also noted that he and Nick have slightly different demeanors when training. "He’s that guy who screams at the gym with his cutoff sleeves,” he joked. "I’m just like, 'OK, cool, bro.’”

Joe was also joined for workouts, which are now a part of his daily routine, by his DNCE band members, Cole Whittle, JinJoo Lee and Jack Lawless. "Cole does a lot of boxing, so we sometimes mess around and hold pads for each other,” Joe explained to Us. “We all kind of had goals that we wanted to train for.”

No amount of hard work, however, could save Joe from the reality of actually stripping down on set. “You’re standing in your underwear all day and having a casual conversation during breaks, but you just kind of forget, 'Oh, wait, I’m just standing in my underwear right now. This is so awkward.’” Luckily, his campaign costar, McKinney, who Joe met for the first time that day, was "a really fun girl and really professional.” Clearly the two enjoyed working together, as she later went on to star in DNCE’s “Body Moves” music video. "I think it’s important in those settings, If you’re going to be that close and that intimate with somebody that you just met,” Joe reasoned, "you want to make sure that you get a good vibe."

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