"I obviously had a very, very good trainer," the "True Blood" star, 39, told E! News. "I basically had him and a nutritionist. I've never done anything like this before, this extreme. I was just really nervous. It was a big challenge and I didn't know how my body would respond to the training and diet."
His routine during filming -- and for several months leading up to the shoot -- included "a lot of chicken cutlets and crunches," he said with a laugh.
The Swedish star's trainer did allow him one day to cheat.
"He was like, 'Alex, let's go for lunch,' and we went to this Italian restaurant in Notting Hill and sat down and he was like, 'Order what you want," Skarsgard recalled. "I started crying. He was like, 'Have pasta,' and I had it and then he was like, 'Are you still hungry?' I was like, 'Yeah!' He was like, 'Have a pizza then, have some tiramisu.' It was the best meal of my life."
Skarsgard's dedication paid off. His impressive physique, and especially those abs, are on full display in the "Legend of Tarzan," which debuts July 1.
As for swinging through the trees, Skarsgard left most of the "crazy stuff" to a trapeze artist with 20 years of experience.
"I took off and did a little bit of it and then I sat in my chair with my coffee," Skarsgard told E! News. "He did the amazing stuff and then I came back and did the landing."