To play a boxing star in “Southpaw,” Jake Gyllenhaal had to transform himself into a bulked-up bruiser who could convincingly go twelve rounds with the fiercest and meanest guys in the ring.

Complicating matters, the actor was a few months removed from shooting “Nightcrawler,” last year’s highly acclaimed thriller that found him playing a gaunt crime scene cameraman seriously in need of a moral compass and a Cheeto. But hitting the gym for hours a day wasn’t the biggest challenge he faced, Gyllenhaal confessed at Monday’s New York premiere of “Southpaw.”

“I didn’t know how to box,” he told Variety. “The getting in shape part was the aftermath and the result of learning how to box and learning a language and a fluency. As I got more fluent, my body transformed. I spent months on my jab. I spent months on my footwork.”

In the process he discovered he had a passion for the “sweet science.” Shooting may have wrapped, but he’s still training.

Boxing will be a part of my life from here on,” said Gyllenhaal, adding, “What I was amazed by was the grace and the sensitivity that’s needed to be a great fighter. People say, ‘Oh, boxing is brutal,’ and this and that, and I say, ‘You know what else is brutal is ballet.'”

“Southpaw” finds Gyllenhaal playing a former champion who loses his career and his daughter after the murder of his wife (Rachel McAdams) sends him into a downward spiral. With the help of a trainer (Forest Whitaker), he readies himself for a return to boxing.

The intensity that Gyllenhaal brings to his roles and his chameleonic approach to acting is what convinced “Southpaw” producer Peter Riche that he could pull off the part.

“There’s a great line in the movie when he says to Forest, ‘I will give you my everything,'” said Riche. “That’s what Jake did.”

McAdams was also impressed by Gyllenhaal’s commitment to the part and his generosity to the rest of the cast.

“He’s always bringing, you know, different material to work and poetry and music — anything that might spark something for someone else,” she said.

Just as Gyllenhaal worked with real life fighters, McAdams immersed herself in the lives of boxers’ families. She discovered that a ringside seat can be just as draining as taking the blows.

“You never feel fully safe,” she said. “When you’re hot, everybody’s there, and then the second you lose a fight it’s like everybody goes away.”

“Southpaw” bows in theaters July 24.

(Pictured: Jake Gyllenhaal, 50 Cent and Rachel McAdams at the New York premiere of “Southpaw”)