LOS ANGELES — There were moments while shooting the crime tale Pain & Gain when stars Dwayne Johnson and Mark Wahlberg would need a serious reality check. Take the scene in the Michael Bay-directed film (opening Friday) where a Miami gang of gym-rat muscle-heads tries to cover up a murder by chopping off the victims’ hands and barbecuing them until they are untraceable.

Even the morning before shooting, Johnson had a hard time getting his head around the concept.”It was one of those moments where I would wake up understanding that I am going to shoot the craziest scene of my whole career, that involves cooking hands on a grill,” says Johnson, 40. “Then I went to bed praying for forgiveness. And I would wake up and do the whole thing again. As we wrapped for the day, I would think, ‘I can’t believe that really happened.’ “It did. Pain & Gain does take some Hollywood license. But the dark, dark comedy about the exploits of Miami’s steroid-and-stripper-loving Sun Gym gang, which kidnapped and murdered in the 1990s, is based on a true story.

“We may have taken a few creative liberties here and there,” says Wahlberg.”But there are parts where you sit there and watch the movie and you go ‘That’s just insane, that’s absurd,’ ” says Johnson”And then you’re reminded that it’s a true story,” adds Wahlberg.

In 1999, a three-part series in the Miami New Times exposed the downright bizarre details of the case — from the multiple, bumbling attempts to kidnap a wealthy Florida businessman, to the clumsy yet brutal way the gang members attempted to kill their victim after he signed away his fortune — including setting him on fire and running him over … twice.

While the crimes were remarkable in their weirdness, Bay says he was more intrigued by the message the story told about appreciating what you have.”These people were looking for the American dream in all the wrong ways,” says Bay, 48. “In the muscle-building world, guys are just never happy. It’s like if you got a little bit bigger, you would be happier.”

Johnson and Wahlberg proved to be the ideal dramatic and physical specimens for the parts. But Johnson, a popular pro wrestler who is nicknamed “The Rock,” was reticent about entering the twisted R-rated tale. His role of Paul Doyle, a composite character, was a significant departure for the action star. It took a thoughtful letter from Bay to persuade him.

LOS ANGELES — There were moments while shooting the crime tale Pain & Gain when stars Dwayne Johnson and Mark Wahlberg would need a serious reality check. Take the scene in the Michael Bay-directed film (opening Friday) where a Miami gang of gym-rat muscle-heads tries to cover up a murder by chopping off the victims’ hands and barbecuing them until they are untraceable.

Even the morning before shooting, Johnson had a hard time getting his head around the concept.”It was one of those moments where I would wake up understanding that I am going to shoot the craziest scene of my whole career, that involves cooking hands on a grill,” says Johnson, 40. “Then I went to bed praying for forgiveness. And I would wake up and do the whole thing again. As we wrapped for the day, I would think, ‘I can’t believe that really happened.’ “It did. Pain & Gain does take some Hollywood license. But the dark, dark comedy about the exploits of Miami’s steroid-and-stripper-loving Sun Gym gang, which kidnapped and murdered in the 1990s, is based on a true story.

“We may have taken a few creative liberties here and there,” says Wahlberg.”But there are parts where you sit there and watch the movie and you go ‘That’s just insane, that’s absurd,’ ” says Johnson”And then you’re reminded that it’s a true story,” adds Wahlberg.

In 1999, a three-part series in the Miami New Times exposed the downright bizarre details of the case — from the multiple, bumbling attempts to kidnap a wealthy Florida businessman, to the clumsy yet brutal way the gang members attempted to kill their victim after he signed away his fortune — including setting him on fire and running him over … twice.

While the crimes were remarkable in their weirdness, Bay says he was more intrigued by the message the story told about appreciating what you have.”These people were looking for the American dream in all the wrong ways,” says Bay, 48. “In the muscle-building world, guys are just never happy. It’s like if you got a little bit bigger, you would be happier.”

Johnson and Wahlberg proved to be the ideal dramatic and physical specimens for the parts. But Johnson, a popular pro wrestler who is nicknamed “The Rock,” was reticent about entering the twisted R-rated tale. His role of Paul Doyle, a composite character, was a significant departure for the action star. It took a thoughtful letter from Bay to persuade him.

“Dwayne was scared of the character, so I literally had to get him off the edge,” says Bay. “I told him, ‘Dwayne you have to trust me on this.’ “Once on board, Johnson played Doyle’s moral fall to the hilt. Portraying Doyle’s spiral into cocaine use had its own challenges, as Johnson had to fake the frequent snorting with powdered milk or artificial sweetener.

“My character was taking mounds of it, and after a while, it burns,” says Johnson. Wahlberg was able to tap into his charming ringleader character, Danny Lugo, by recalling his porn star character from 1997’s Boogie Nights.”(Lugo) is kind of the grown-up, confident, steroid-headed version of that character,” says Wahlberg.”There is nothing worse than idiots with confidence,” adds Johnson. “And that’s what these guys were.”Bay plays up the dark comedic aspects of the crimes in Pain & Gain. In one scene, Lugo, whose criminal knowledge comes from extensive movie-watching, sets up one kidnapping attempt with the perps wearing Halloween ninja outfits.

When together, Johnson and Wahlberg can play along in the idiot moments with the best of them. During a rooftop lunch at the posh London Hotel here, they throw out lowest-common-denominator retorts at each other whenever possible. As Wahlberg leans over a healthy meal (chicken, black beans and a salad featuring his own bottle of fat-free Italian dressing), he goes into TMI detail about the effects it will have on his digestive systemJohnson just rolls his eyes and laughs before insisting that his co-star had a Rock poster above his bed to motivate him in the gym.

The two showed their own particular style in the workouts. Wahlberg would hit the gym with his ever-present posse of boisterous friends to help motivate him. “There would be weightlifting competitions with me and my friends. And there would be bets,” says Wahlberg. “People would literally be losing their paycheck.”He admits it was effective, but not healthy in the long run to “max it out in the gym five days a week.” Meanwhile, Johnson could not be persuaded to join in the macho antics, despite Wahlberg’s prodding.

“We’d always try to goad him, trying to see what he could lift,” says Wahlberg. “But it wasn’t about the ego with him. He’s obviously a lot more smart when it comes to doing it the right way.” The right way included staying away from steroids to get huge for the part. Johnson has acknowledged experimenting with steroids while playing football at the University of Miami (“we didn’t know what we were taking or if anything worked,” he says).

But he insists he didn’t use drugs to prepare for Pain & Gain.”Oh, no,” says Johnson when asked directly, looking at Wahlberg. “How about you? You did all kinds.” “What?” says Wahlberg, snapping to attention. “Steroids? No. I don’t (mess) with those.”

Yet Wahlberg was able to whip himself into good-enough shape that he showed off his Calvin Klein underwear for the first time since his ripped abs created a global sensation in the 1990s. Wahlberg insists Bay persuaded him to shoot the scene”I did not want to do it, that was that time period,” Wahlberg grumbles. “Michael (Bay) was like, ‘Come on, man.’ And I’m game.”

Better to stay in Bay’s good graces since Wahlberg will next be shooting Transformers 4, due out in 2014. Wahlberg’s training will not be needed for the role, but he’s still working out extensively.”I prefer to feel the way I feel now,” he says. “I just feel better waking up early and exercising.” Johnson will continue to push his physical limits while preparing for the title role in Hercules: The Thracian Wars. (No word on whether his recovery from Monday’s emergency hernia surgery will affect the 2014 opening.)

He proudly pulls out his smartphone to show Wahlberg the latest picture of the set. “You’re going to love this,” he says. “This is me a couple of days ago.” “Skimpy, I love the leather,” says Wahlberg, looking at the minimal costume. “Skimp it down a little more.” Johnson is working out with heavy cardio training to lean down for the part, hitting the gym twice in the morning before even heading into work. “This will be the biggest prep physically and diet I’ve ever done,” says Johnson. “Our version is going to be the definitive one. Darker. And a little more animal.”