MACK in the middle of Mark Wahlberg’s cavernous home gym here, amid the racks of weights and rows of cardio equipment, is the emblem of a career-long ambition: a regulation-size boxing ring.

For as long as he has been a movie star, Mr. Wahlberg, 38, has wanted to play a boxer. He has come close a few times. He had the lead role in a never-made biopic of the middleweight champion Vinnie Curto and was briefly attached to “The Black Dahlia,” the mystery noir whose hero is an ex-pugilist. Mr. Wahlberg now finally has a boxing movie almost in the can, and the bonus is that it’s an especially personal one. “The Fighter,” of which he is also a producer, tells the life story of the Lowell, Mass., boxer “Irish” Micky Ward, one of his childhood heroes.

Mr. Wahlberg has been training for “The Fighter” for more than three years. The morning we met, he had already completed a four-hour workout that started at 6:30 a.m. Mr. Wahlberg said it was important for him to show “the most realistic boxing ever in a film,” which meant sparring with real fighters. The general idea: “Let’s try not to kill each other, but definitely get in there and take some shots.”

The demands on his body have only intensified as the shoot has wound to a close. He gained nearly 30 pounds for a handful of scenes filmed in March that show a retired, out-of-shape Ward. When feedback from a rough-cut screening suggested a few more close-ups for the climactic fight, Mr. Wahlberg had to lose that extra weight quickly — hence a stepped-up training regimen and a strict low-carb diet. The previous day, he said, he had taken his family to brunch at the Four Seasons in Los Angeles, and while everyone else feasted on prime rib and pancakes, he sipped water.

But he’s not complaining. “I don’t mind working hard,” he said. “Look how fortunate I’ve been with my hard work.”

As Mr. Wahlberg would be the first to admit, he now has an acting and producing career that few could have foreseen back in the mid-’90s when the rapper, teen idol and underwear model known as Marky Mark decided to reinvent himself in Hollywood. “I feel like I’ve snuck in the back door,” he said.

This year Mr. Wahlberg has an eye on both the summer box office and the awards season. “The Fighter,” which reunites him with the director David O. Russell, who steered him toward two of his most affecting performances, in “Three Kings” and “I ♥ Huckabees,” is being readied for a year-end release. But first, he further develops his comic persona, recently on view in “Date Night,” in “The Other Guys” (Aug. 6), a buddy-cop action spoof in which he and Will Ferrell play New York Police Department underachievers.

It was Mr. Russell, a close friend of Mr. Wahlberg’s, who introduced him to Adam McKay, the director of “The Other Guys.” “Mark has a great capacity for comedy,” Mr. Russell said, “but it’s a comedy that comes from being very real and intense.”

Mr. McKay, the man behind the goofball absurdism of “Anchorman” and “Talladega Nights: The Ballad of Ricky Bobby,” said that Mr. Wahlberg’s character in “The Other Guys,” who has “an anger disorder but is also very vulnerable,” drew on the actor’s ability “to play wounded and betrayed in a really funny way.”

Even though “The Other Guys” required him to think on his feet — Mr. McKay and Mr. Ferrell’s preferred method is nonstop improvisation — Mr. Wahlberg said he was able to stay within his “comfort zone,” which he described as “my commitment to playing it as real and as straight as possible.”

Mr. Wahlberg is the rare actor who’s at ease in both maximalist and minimalist modes. He can be compelling while acting up a storm (a hotheaded detective spewing salty tirades in“The Departed,” a conscience-stricken firefighter ranting against petro-capitalism in “Huckabees”) or when seeming to do very little (the most famous scene in “Boogie Nights,”in which a drug deal messily unravels, peaks with a nearly minute-long close-up of his dumbfounded expression).

In either case his great gift as an actor is his straight face. He wears it not as a mask of deadpan irony but as a mark of deep sincerity, which can be a source of comedy or pathos, sometimes both at once. He doesn’t often get to stretch, and he has played more than his share of cops and criminals (“I’m still waiting for a call for an English period piece,” he said). But he is almost without fail the most convincing thing about any movie he’s in.

His willingness to plunge in body and soul serves Mr. Wahlberg in ambitious, demanding films: Mr. Russell’s madcap farces, Paul Thomas Anderson’s “Boogie Nights,” the near Greek-tragic family dramas of the writer-director James Gray (“The Yards,” “We Own the Night”). And it helps even when the movies are middling or worse. His faith in the material elevates the sports-underdog clichés of “Invincible,” grounds the doomsday horror of “The Happening” and cuts through the New Age mysticism of “The Lovely Bones.”

A popular “Saturday Night Live” sketch from 2008, “Mark Wahlberg Talks to Animals,” featured Andy Samberg as Mr. Wahlberg earnestly addressing a barnyard menagerie (“Hey, donkey, what’s going on?”). At first glance it’s a dopey non sequitur skit, but it’s also a slyly backhanded tribute to an actor who can commit to just about anything.

“I know no other way,” Mr. Wahlberg said of his approach. “I’ve seen a lot of people try a lot of tricks, and I don’t feel so comfortable with that. The only way I can do it is to believe it.”

Mr. Russell pointed out that this heartfelt intensity is harder, and braver, than it looks. “In this world that can be a very painful place, there can be a tendency to guard yourself,” he said. “You can use irony or cynicism to do that, and so sincerity becomes a great act of courage.”

The winning directness that is now Mr. Wahlberg’s hallmark is perhaps the flip side, or the grown-up version, of the keep-it-real imperative that guided him as a rapping homeboy and, before that, as a teenage thug in working-class Boston. (He spent six weeks in jail for assault when he was 17.)

In his early roles, the flexed pectorals and come-hither scowls of Marky Mark still imprinted in the pop-culture consciousness, he went through the bad-boy motions, playing Leonardo DiCaprio’s hoodlum sidekick in “The Basketball Diaries” and Reese Witherspoon’s seducer-stalker in “Fear.”

Then came “Boogie Nights,” in which his wide-eyed busboy finds a calling and a community after assuming the nom de porn Dirk Diggler. Mr. Wahlberg went into the film with some trepidation, he said, worried about what “the guys in the neighborhood” would think. But playing someone as guileless as Dirk was liberating.

“It was nice to be that guy, because for a long time I had my guard up,” he said. “I had to be this guy who was perceived as tough, and if you’re not that guy, then you’re the victim.”

Mr. Wahlberg has not shied from roles that echo his troubled past. Mr. Gray said that for “The Yards,” his drama about corruption among Queens subway contractors, he had asked Mr. Wahlberg to play the smooth operator Willie (a role that eventually went to Joaquin Phoenix), but Mr. Wahlberg wanted the less showy part of Willie’s childhood friend Leo, an ex-convict. “He knew the fear and the insecurity and the desire for redemption,” Mr. Gray said in an e-mail message, adding that Mr. Wahlberg’s best work “always reflects a life that is lived.”

“Sometimes I think he’s an actor slightly out of his time,” Mr. Gray continued. “He feels to me like our answer to John Garfield.”

Garfield, a vivid tough guy who played boxers more than once, was among the actors Mr. Wahlberg watched as a kid with his father. Mr. Wahlberg recalled that when “Fear” was reviewed in The New York Times, he was compared to Garfield and Robert Ryan. (“I knew who those guys were,” he said proudly.) While he was always drawn to old-Hollywood actors with a “blue-collar, regular-guy quality to them” (a poster of the James Cagneyclassic “Angels With Dirty Faces” hangs in his kitchen), he never thought of acting until the director Penny Marshall offered him a small part in the 1994 comedy “Renaissance Man.”

“For a long time I’d been acting in my life anyway, whether conning my way into something or out of something,” he said. “I was always a fairly good salesman, and a fairly bad liar, so if I believed it, I could do a good job of convincing somebody else.”

Mr. Wahlberg has long been candid about his teenage delinquency. He set up a youth foundation in 2001, and he talks about being a role model for kids from similar backgrounds “who can be creative and artistic and be considered cool,” he said.

Increasingly the rising-star exploits that inspired “Entourage,” the popular HBO series he produces, also seem to come from a previous life. Mr. Wahlberg described himself as above all a devout Roman Catholic, a devoted husband and father of four. “The first thing I do every day when I leave my house,” he said, “I go to church, man, get down on my knees.”

The buddies who inspired the characters in “Entourage” are still in his life — he often practices his lines with them — but the only entourage in evidence at the Wahlberg residence that day was a small contingent of maids and nannies. His Mediterranean-style home, where he has lived since his bachelor days, has guy-pad luxuries like a grotto pool and a putting green, but with the toys strewn around the grounds, it also looks very much like a four-child household.

Before adjourning to the guest house for the interview, Mr. Wahlberg checked in on the family lunch in the kitchen, where his youngest, 3-month-old Grace, dozed in a baby swing. He interrupted the conversation to help his wife, Rhea Durham, open the electric gate (the power was out) as she prepared to take their two older children to a matinee of“How to Train Your Dragon.” (“No Lady Gaga,” Mr. Wahlberg told his eldest, Ella, 6, as they drove off; she had come home from school singing age-inappropriate lyrics.)

Mr. Wahlberg’s producing career continues to flourish: “Boardwalk Empire,” a series about Atlantic City in the 1920s, is scheduled to start on HBO this fall, with a pilot directed by Martin Scorsese. But no amount of success — not even the Oscar nomination he received for “The Departed” (best supporting actor) — has made him feel fully at home in the industry. His work ethic is bound up with the sense that he still has something to prove. “I appreciate every opportunity I’ve been given,” he said. “I want to show up early. I want to be the most prepared.”

Preparation is an inadequate word for the herculean effort that has gone into “The Fighter,” a passion project that has an air of autobiography. “There’s a lot Micky and I have in common: our willingness to work hard, to do whatever we have to do to achieve our goals,” Mr. Wahlberg said. He described the film as a tribute to his and Mr. Ward’s shared origins, a story about “that world and how difficult it is to make it out of there, and how important it is to people to have somebody to root for.”

Mr. Gray recalled that while making “The Yards,” he often found Mr. Wahlberg rereading pages that had already been shot. Mr. Gray said that when he pointed out they had moved on to other scenes, Mr. Wahlberg would “look at me like I was nuts.”

His response had the ring of a credo: “Jim, I always need to know where I was, and where I’m headed.”