For Mark Wahlberg\’s portrayal of fighting champ \’Irish\’ Mickey Ward, he said he had to train four and a half years to ensure the movie\’s authenticity and success.[/media-credit]

Everybody loves an underdog. Since biblical times when David downed Goliath, people have been drawn to the little guy, no matter whether it’s in sports, life or even a little of both.

“The Fighter,” an upcoming boxing biopic from director David O. Russell (“I Heart Huckabees”), tells the true story of underdog “Irish” Mickey Ward (Mark Wahlberg, “The Other Guys”) who went from being a stepping-stone in the ring to a title contender. At a recent press conference in Los Angeles, Russell and the stars of “The Fighter” discussed their latest film, weight loss, Boston accents and cast chemistry – both onscreen and off.

While his character’s story is the real underdog tale, Wahlberg had to overcome some obstacles of his own in order to finally bring Mickey’s life to the big screen. Yet, he continued to train for the role and even did double duty as producer “out of sheer desperation for getting the movie made.”

“Had somebody said you have to train four and a half years to make this movie I would have said absolutely not,” Wahlberg said. “But I continued to do it, because I figured if I stopped I would be giving up on the movie, and I never wanted to do that.”

Wahlberg eventually brought in Russell, a past collaborator and good friend, to direct, which resulted in a previously untapped level of humor and emotion. However, with Wahlberg taking on more of a vocal role than usual as producer, the relationship between the two wasn’t quite the same as it had been in the past.

“When I first met [Mark] he was a 26-year-old mumbling kid off of ‘Boogie Nights’ and we made this movie, and he was like Boardwalk Empire builder or the Godfather,” Russell joked.

“Shit happens, dude. I’m a hustler. I’m from the fucking street, baby. I gotta make it happen. Nothing comes easy for me,” Wahlberg replied.

Although “The Fighter” is a boxing film, the heart of the story is Mickey’s relationship with those around him, especially his older half-brother and trainer, Dickie Eklund, a former boxer whose career ended prematurely due to a crack addiction. Wahlberg knew Christian Bale was the man for the job after a chance encounter at their daughters’ school.

“I thought there is the guy who’s not scared to play this part,” Wahlberg said. “Everybody loved the idea of it, but nobody really wanted to commit and go there. He’s a fearless actor, and he just responded immediately.”

Known for his commitment to playing shockingly gaunt characters in “The Machinist” and “Rescue Dawn,” Bale once again had to slim down to realistically portray a crack addict, but admitted there aren’t a whole lot of secrets to his weight loss regiment.

“I was just running like crazy. I could just run for hours on end. Usually I always say I do a lot of coke whenever I lose weight. I’m not sure if it’s so funny for this movie to say that though,” Bale quipped.

To truly capture their characters’ personality, though, the cast spent time with the family – Wahlberg even put up Mickey and Dickie in his house for some time during rehearsal – giving them an opportunity to share their stories and add input, even if it wasn’t always welcomed.

“There were a couple of times I had to physically restrain Dickie from going and landing one right on David,” Bale said.

In fact, it wasn’t until after meeting the family that Melissa Leo (“Frozen River”) believed she could play the role of Alice Ward, the mother of Mickey and Dickie.

“Although it sounded like an extremely exciting and interesting project, I still had a lot of doubts,” Leo said. “But upon meeting Alice Ward, I saw immediately my maternal grandmother in her and knew then that I have her in here somewhere.”

Because the story takes place in the city of Lowell, Mass., the actors also had to deal with the oft-dreaded Boston accent. A Boston native, Wahlberg admitted it was a lot harder to get rid of the accent then it was to get it back, but it wasn’t nearly as easy for an English actor like Bale.

“Mark was a great deal of help in that he wouldn’t ever say anything, but he’d just get a certain look on his face when you said something that you just knew that wasn’t it,” Bale said. “I also really had to tone down [Dickie’s] natural rhythm and voice because if I’d done it exactly like Dickie we would have needed subtitles.”

Perhaps the most surprising addition to the cast, though, is Amy Adams (“Leap Year”), who was looking to break type as Mickey’s girlfriend and future wife, Charlene.

“When I got the role, David informed me that I looked like a girl who couldn’t punch which made me want to punch him,” Adams said.

Russell claimed, however, he knew from the start that Adams was going to kill the role.

“Charlene is a tough bitch and Amy has that fierceness in her, but Amy also brings a great deal of emotion in her eyes, so you have that great cocktail that I find so interesting.”

Coincidentally, Wahlberg had actually previously met with Adams to talk about her starring in another movie – one that he’s not quite as proud of.

“It was a really bad movie that I did. She dodged the bullet,” Wahlberg joked. “I don’t want to tell you what movie … alright, ‘The Happening’ with M. Night Shyamalan. Fuck it. It is what it is. Fucking trees, man. The plants. Fuck it. You can’t blame me for not wanting to try to play a science teacher. At least I wasn’t playing a cop or a crook.”

Fortunately, not having the opportunity to star beforehand in a film with Wahlberg didn’t affect Adams onscreen chemistry with the former underwear model.

“I mean how hard is it to pretend that you are attracted to that,” Adams quipped. “I’m such a good actress, right”?

Although “The Fighter” doesn’t include some of Mickey’s more famous fights, including his three bout series with Arturo Gatti, Russell believes the underdog story of how he got there is legendary in itself.

“And we are doing those fights in the sequel,” Wahlberg said. “We’ll do four more ‘Fighters.’ We will do the first Gatti fight in the sequel and then we’ll do the second one in the third installment and the fourth and final one will be Mickey …”

“In Russia,” Russell added.

“The Fighter” opens in select theaters on Friday, December 10.