LOS ANGELES - You don't earn the nickname "The Rock" unless you're pretty ripped. But even Dwayne Johnson admits he was impressed on the first day of filming Pain & Gain last year to see the bulked-up version of co-star Mark Wahlberg.
"I thought, 'Wow, he really packed on some muscle,' '' Johnson says.
So that makes twosuper-sized actors for the film, opening Friday. Both Johnson, 40, and Wahlberg, 41, turned back time to pump up to career-high sizes to play two Miami bodybuilders - guys with under-utilized brain muscles, but massive physiques.
"It wasn't easy. It's a lot of hard work," says Johnson, who's also a pro wrestler. "And a lot of protein."
The Michael Bay-directed film is based on the true story of Miami's Sun Gym gang, which hatched a twisted abduction scheme to get rich in the 1990s - leading to extortion and murder. Bay championed the project for close to a decade before pulling together the right team. The acting chops came first over physique, but a big body was key for the role.
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He zeroed in on Johnson, who added 12 to 15 pounds of muscle to his 6-foot-4 frame to push over 250 pounds, and Walhberg, whose 5-foot-10 body started at a lean 165 pounds and later maxed at 212 pounds.
"I knew Dwayne was already there. And with Mark it was like, 'Can you do it?' and he said 'Ya!' " says Bay. "Mark was like a machine. He was like 'I'm going to be the biggest I can be.' They were both fully committed. That's as big as (Johnson) has ever been."
There were some perks to the gig. Wahlberg says he enjoyed the building-up phase where he was forced to have up to 10 meals a day.
"The only part where that's not fun is when you've gone to bed at 10 o'clock after a big meal. And then at 2 o'clock you wake up to eat again and you're still full from the last meal," says Wahlberg, motioning to his throat. "You can feel the food right here. When you burp, pieces of food come up."
He spent 12 weeks on the diet while pumping iron for the role of Sun Gym ringleader Danny Lugo.
"For months afterwards I was sore," says Wahlberg, who re-aggravated past shoulder injuries. "The toughest part for me was realizing that I'm not a kid anymore at 41. Trying to lift heavy weights and pack on pounds and muscle was difficult."
Johnson, who had emergency surgery Monday for a hernia suffered in a WWE match last month, says that he had to moderate his Pain workouts by "training smarter."
"When you're young, you think you're invincible," says Johnson. "When you're older, you have to start listening to your body."
Coming off an impressively sized starring role in G.I. Joe: Retaliation, Johnson's additional muscle, via the workouts and a strict diet, looks like more.
"Onscreen, that small amount of extra pounds translates to 50 pounds, making my character look like an 800-pound silverback gorilla just released from jail," he says.
Wahlberg says he never tried to out-muscle his co-star, whom he dubs "a freak of nature."
"You gotta know your limitations. For me, it was about being the best I could be," says Wahlberg. "But it's hard to be comfortable standing next to this mountain of a man when he's playing a bodybuilder."
(Copyright © 2013 USA TODAY)