While a member of the Jonas Brothers, Nick Jonas was certainly the smallest of the group.

Nowadays, the youngest of the group has gotten muscular, and he showed off his body on Details magazine. Jonas gained 15 pounds of muscle for his role in "Kingdom."

He worked with a trainer and worked on strength and fight training three times a day to prepare for the role. He also ate about 4,200 calories, where he used to eat about 3,000.

"I got pretty Hulk-ish," he said. "Breakfast was usually an omelet with veggies and cashew cheese, a turkey-meat patty and some fruit; lunch and dinner would be lean protein, veggies and nuts."

He said that he wasn't drinking while he was training.

After shooting "Kingdom," he kept it "classy with Coors Light." The actor also lost 10 pounds of muscle by decreasing his caloric intake to about 2,500.

The actor explained that he, just like many people, dislikes the gym.

"But I hate going to the gym," he said. "I think everybody hates cardio to some degree. I'm not the kind of person who wakes up in the morning and can't wait to work out. I kind of dread it, actually. I see it as a necessary evil."

Jonas, who is in a relationship with Miss Universe 2012, Olivia Culpo, talked about his girlfriend feels about his sex scenes. He said that the two have been happily together for about a year and a half, and she understands that it is work.

He called it supportive and said that being honest and trusting each other is what helps the two's relationship work.

He recently posed in his underwear in for Flaunt magazine, which was very reminiscent of Mark Walberg's days as an underwear model. But Jonas said he wasn't sure if he could commit to the gig full time.