Fans of Dwayne Johnson have grown to expect a certain evolution from the characters he portrays. If his character is not a hero at the start, he is heroic in the end. If he isn’t a leader, he has leadership qualities by the final credits.

The wrestler-turned-actor has played a lot of likable characters. And the 40-year-old Johnson, always charismatic and affable, knows this. It was the reason he had second thoughts just before filming began a year ago on director Michael Bay’s new movie, “Pain & Gain,” which opens today.

As bodybuilder Paul Doyle — an ex-con involved in a brutal kidnapping, extortion and murder scheme in Miami — Johnson would play the most loathsome character of his acting career.

Johnson’s dilemma? The script was so good he couldn’t resist. The part of Doyle possessed more richness and complexity than Johnson normally has an opportunity to play.

"About a week out, I got cold feet," Johnson said in a phone call from Miami last week, before he underwent surgery to repair hernial tears suffered during a Wrestlemania event April 7. "My biggest concern was, will audiences be happy with this and seeing me involved in such a horrid crime and sniffing mounds of cocaine off a stripper?"

The film, a dark action-comedy that also stars Mark Wahlberg, was based on crimes committed in the 1990s by a group of bodybuilders led by Daniel Lugo. Johnson remembers hearing about them at the time because he played on the University of Miami football team.

The crimes go wrong in such horribly absurd ways that Johnson had to keep reminding himself it really happened.

"It is absurd and incredible and unbelievably insane," he said. "They had done some horrible things. They killed and dismembered and cooked body parts to get rid of the fingerprints."

Mired with this quandary, Johnson called Bay, who had given the actor a copy of the script eight years earlier. The director, whose action-packed films include the "Transformers" series, "Armageddon" and "The Rock," told Johnson his character would be the audience’s conscience, Johnson said. When filmgoers want empathy, they will find it in Johnson’s character, Bay told the actor.

"Then he sat down and wrote a letter to me," Johnson said. "He was incredibly articulate and empathetic and direct. And the spirit of the letter was: Will Smith had to trust me in ‘Bad Boys.’ Nicolas Cage had to trust me in ‘The Rock.’ Bruce Willis had to trust me in ‘Armageddon.’ I need you to trust me in ‘Pain & Gain.’"

The part of Doyle, which also required 15 pounds of new muscle, was a challenge, Johnson said.

"His wiring and constitution and makeup are nothing like I am," Johnson said. "He is weak and easily influenced. He has no family and no ideology of family or love. He looks for acceptance and finds it in two very shady characters. When he goes off the deep end, he really goes off the deep end. His cocaine use is epic. It is epic in a very disturbing way."

Johnson is just coming off two big films, "The Snitch" and "G.I. Joe: Retaliation." The sixth installment in the "Fast & Furious" franchise is in post-production with another rumored to be in pre-production. And he will have the starring role in "Hercules: The Thracian Wars," the shooting of which will begin in Budapest this summer.

Johnson has worked four years without any real time off, but it has given him box-office star power to pick and choose his roles.

It’s a long way from a career as a professional wrestler and even further from his time in Hawaii, where he lived on and off through his early high school years.

He’s appeared in a range of parts, from action heroes in "The Rundown" and "Walking Tall" to the goofy and comedic in "Be Cool" and "The Tooth Fairy." In the 2010 dark drama "Faster," he was an ex-con hunting down those responsible for his brother’s death.

"When I first started, in ‘The Mummy Returns’ (2001), I wanted to be in this position: to have choices, leverage, diversity and making movies people could enjoy," he said.

But saying yes to Bay became what Johnson calls a watershed moment in his acting career.

"It has become my defining role," he said. "Whether the movie does wonderfully or it doesn’t, it was just a moment in time in a 13-year career where I had an opportunity to abandon every quality I look for in a character and every quality that audiences generally want to see from me."