The first photo that the world saw of Jake Gyllenhaal in Southpaw (out July 24) made the actor look like an entirely different person than the man last seen as a rail-thin psychopath in Nightcrawler. Bruises had replaced the dark circles on the Golden Globe nominee’s face, and every other inch of him was tatted, ripped, and muscular. This wasn’t Lou Bloom. This was light heavyweight champion Billy Hope.

That’s why it was so shocking to hear that Gyllenhaal only gained an additional 10 pounds for the boxing role. “I was just in a different kind of shape,” Gyllenhaal says, putting it mildly.

Gyllenhaal’s journey to authentic light heavyweight status began five months before cameras started to roll, when director Antoine Fuqua offered him the role. Known for his commitment to his characters, especially when it comes to the physical aspect, Gyllenhaal knew that there was only one way to convince himself and the audience.

“There’s a tremendous physical aspect about getting into shape or learning how to box, but for me, you can’t play a boxer and just look like a boxer. That doesn’t make sense to me, in my mind,” he says. “To play a boxer, to play any role, you have to believe that you can exist in that world, or else your confidence in the scenes and your interactions with every character—particularly in a movie like this where you’re working with real fighters—is a waste of time.”

Despite what the stills from Southpaw might suggest, all Gyllenhaal did to become Billy Hope was… box. No chopping wood in the Russian wilderness, or wrestling bears: He just boxed. A lot.

“I don’t know if that makes sense, but I just remember two-a-days for five months, and learning as best I could the sport of boxing and have it envelope me and going to many fights—being in that world and just training, training, training,” Gyllenhaal says.

All of that work—at least from Gyllenhaal’s perspective—is not only for him, but for the audience as well. It makes the movie a more believable and ultimately enjoyable experience: “You spend how many thousands of hours in the gym with lots of other people’s sweat and your own everywhere. Then that will be in the movie,” he says. “The audience will feel that. It’s fun. It should be fun.”