Jake Gyllenhaal is prepping for his big fight scene in “Southpaw.” I’m supposed to meet him on a Sunday afternoon in August 2014. But when I walk into a private boxing gym in Pittsburgh, which doubles as director Antoine Fuqua’s production office, it’s deserted except for a man wearing a sleeveless hoodie and baseball cap, sprinting on a treadmill. The man, who has his neck and arms covered in tattoos, waves at me. I’m disoriented, until I spot the familiar grin. Oh, hi, Jake Gyllenhaal.

It’s pouring outside, and Gyllenhaal makes an imploding gesture with his fist. “Thunder is amazing,” he says, as he climbs off the treadmill, only huffing a little. “Do you want to work out with us?” I look down at my jeans and collared shirt. But Gyllenhaal rummages through a duffel bag and offers me his T-shirt and shorts. My mind goes to two places: (a) If a movie star lends you his clothes, it’s bad manners to turn him down, right?; and (b) the last time I boxed was — never. Unless you count Nintendo.

As Hollywood faces a leading-man crisis (with George Clooney, Brad Pitt and Tom Cruise all in their 50s), Gyllenhaal, 34, has quietly emerged as the actor of his generation. Five summers ago, he removed himself from the studio tentpole track (after “Prince of Persia: The Sands of Time”) and started a career reinvention in the indie world. There was his big-hearted cop in “End of Watch,” a trippy pair of twins in “Enemy” and a twitchy detective in “Prisoners.”

After that, he played a crime paparazzo in last year’s “Nightcrawler,” a role for which he lost 30 pounds (and came this close to getting a best actor Oscar nom). In “Southpaw,” which opens today, he portrays a heavyweight boxer who tries to stage his comeback in the ring to regain custody of his daughter.

Back in Pittsburgh, I’m just trying to stay alive. As I re-enter the gym in my borrowed workout clothes, Gyllenhaal and I go through a round of warm-ups. Or more accurately, he does the exercises while I watch. He cranks out a set of dips without breaking a sweat, between sit-ups and pull-ups, and finally assumes a squatting position to flip a 200-pound tire across the width of the gym floor. Then it’s my turn. I try. And try. And try. The tire stays on the ground.

The script for “Southpaw” was originally written for Eminem, who turned it down because he no longer wanted to act in films. Harvey Weinstein, who is distributing the film, suggested Gyllenhaal as a replacement. “I talked to Jake right after ‘Nightcrawler,’ and he was rail thin,” says “Southpaw’s” screenwriter, Kurt Sutter. “I knew he had a lot on the line taking this role, and he wouldn’t let anybody suggest he wouldn’t become this guy.”

Fuqua said that he met Gyllenhaal years ago, and always knew that the movie industry underestimated him. “He’s not little Jake anymore,” Fuqua says. “I think he was feeling that as well.”

Gyllenhaal spent six months training to prepare for “Southpaw,” starting in January 2014. He weighed only 150 pounds, and worked out twice a day (for six hours a day) to build a 175-pound boxer’s frame. According to his trainer Terry Claybon, of the boxing gym Lb4Lb, Gyllenhaal would do 2,000 sit-ups a day, 500 push-ups, 100 dips and 100 pull-ups and run eight miles to up his stamina, since the fighting scenes in the film were shot in real time, with Gyllenhaal actually punching out his opponents for 12 rounds. “He is shooting 14-hour days,” Claybon says. “His stunt double is here, but Jake will not let this guy work. He insists on doing everything himself.”

Claybon starts to wrap my fingers with a white band. He asks if I’ve been working out with a trainer. When I tell him that I have, he scowls. “He must talk a lot,” he says, as he pokes me in the gut. He later tells me that on the first day of the “Southpaw” shoot, before a fight scene in front of 2,000 extras, Gyllenhaal was so unnerved, he kept saying his hands weren’t wrapped right. “I get frustrated and made him get his gloves on,” Claybon says, adding that he always wraps hands correctly. “Once we got out there, after the first shot, we couldn’t get Jake back in the dressing room.”

I doubt that I’ll be a natural in the ring. Anyway, my stance is all wrong. Claybon tells me that my feet need to be positioned at 90 degrees, and he shows me how to jab in the air. I try to flick my wrists like he does, but it looks like I’m pretending to hit someone as opposed to actually following through. I envision myself getting knocked out by the star of “Southpaw.” The ambulance. The hospital bills. Never leaving Pittsburgh.

I asked Gyllenhaal if he’s been injured. “I got two pretty hard hits,” he says. “I got hit pretty hard in my jaw, and I got clocked in the face.” As I try to plot my escape, I’m rescued by Victor Ortiz, Gyllenhaal’s co-star in the film. When the guys start to spar, I decide that it’s time to take an indefinite break, and quickly exit the ring. “Are you OK?” Gyllenhaal mouths. I nod. Boxing is a sport that I’m really good at, as long as I’m the spectator.