At the Church Street Boxing Gym, a basement grotto in lower Manhattan, goads to activity include peeling posters of Marvelous Marvin Hagler, the trainers’ “fighting solves everything” shirts, and the rap soundtrack. Also, there aren’t any chairs. Jake Gyllenhaal, who worked out there twice a day for six months, said that his trainer, Terry Claybon, insisted, “Never sit down, never lean against the ropes, never cross your arms.” Surrounded by boxers who were skipping rope and hitting mitts, the thirty-four-year-old actor wore his Nikes unlaced and had Ray-Bans hooked over the collar of his T-shirt, but he stood erect as a poplar.

In “Southpaw,” which opens this week, Gyllenhaal plays a light-heavyweight champ named Billy Hope, an impulsive mauler. After his wife is shot and killed during his scuffle with another fighter’s posse, Billy loses control—then loses his boxing license, his house, and custody of his daughter. He takes work as a janitor at a gym in return for training sessions with the gym’s contemplative proprietor, Tick Wills (Forest Whitaker), who teaches him how to stop punches with his gloves rather than his face, and, over time, how to be a man.

As Gyllenhaal walked across the gym, a sweaty bantamweight called out, “I saw the trailer. You looked so cut!” The actor, whose regimen included a hundred pullups and a thousand situps per day, thanked him. After a moment, he confided, “A boxer walks into a ring, he looks fit—then they fight. There’s a whole language here that I was trying to learn and respect. Because I had very large doubts about whether I could pull off the behavior.”

He explained, “The first thing I learned was how to drag the back foot.” He skipped through a series of chain-gang lurches. “Never pull the back foot off the ground, or you’ll get tagged. Then ‘shelling up,’ defending like Floyd Mayweather.” He turned half-sideways, hunched his left shoulder and tucked his elbow to protect his chin and ribs, and peeked around his own ramparts. “And slipping the ropes: tedious, tedious footwork, turning side to side so your back remains against a rope running across the ring—totally not instinctual. I hated it.”

Sitting on a medicine ball, he acknowledged the appeal of redemption through savage self-discipline. However, he noted, “In ‘Rocky,’ which I loved for the characters, you had the problem of it being all three-punch combinations thrown from the floor and the actors overreacting to the blows. In a real fight, you rarely register that a boxer has been hit until he falls down.” He stood and put his nose against a heavy bag. “Terry had me get real close, and he taught me it only takes five pounds of pressure to bang someone’s brain into his skull and knock him out. I came in thinking boxing is about aggression and strength, and I learned it’s about grace and movement and accuracy.

“I began, in Terry’s words, as ‘awful,’ and got to ‘moderately interesting.’ Beginners throw a punch from their arms”—he demonstrated, effortfully—“while fighters’ shoulders and torsos separate. Months in, I was suddenly able to have my shoulders pop!” His arms rippled out rapidly, like tentacles. “It’s just so relaxing. It’s a salsa dance.”

In “Southpaw,” Billy himself starts to pop when he seeks out Tick. The scene of their meeting was largely improvised. “I’m standing in the threshold,” Gyllenhaal recalled, “using the doorway as a defense against his challenging words. And I’m carrying, like, fifty pounds of shit in my gym bag—all my boxing gear, three sweatshirts, a map, framed photographs from the house. Rule No. 1 for me is I don’t believe bags in movies should be filled with little ghost turds.” He shrugged, wryly—Method actors.

The room began to fill, and Gyllenhaal found a perch on the stairs. He watched the fighters circle, boxing with shadows. “I wish I could measure, in buckets, how much I sweated here,” he said. “Four shirts every time. Conservatively, I lost eight hundred pounds trying to join my body to my mind. Think how much in a fighter has been consumed and consolidated and excreted!” He laughed, acknowledging that his words hadn’t quite attained five pounds of pressure. “It’s not as beautiful a thought as it would look like translated to a Gatorade ad. But still.” ♦