Go Behind the Scenes With Matt Damon and Bradley Cooper's Trainer

"Film is forever" is something of a mantra in Hollywood. Endure whatever you need to endure to get the shot, because it's going to last. This could mean repeatedly hurling your body onto the hood of a car, shooting take after take in the freezing cold or staying up all night waiting to get the perfect sunrise shot. To Jason Walsh and his clients, though, it means hill climbs. Also: sit-ups, push-ups and pull-ups. Thousands of them. Waking up at 5 a.m. is usually involved, too. Oh, and the camera hasn't even started rolling yet.

This is because Walsh is a celebrity trainer who is routinely tasked with transforming an actor's phyiscal appearance, often radically, in a matter of months. He got John Krasinski and Bradley Cooper bulked up for 13 Hours and American Sniper, he got Justin Timberlake toned for Friends With Benefits and Justin Theroux looking like a cop for The Leftovers. Walsh's most notable client, though, is Matt Damon. They first met when Damon needed to shed the pounds he put on for 2009's The Informant, and Walsh recently put the Oscar winner through a number of exhaustive workouts to get him looking cut for The Martian, The Great Wall, which is due out in 2017, and the latest installment in the Bourne franchise, which hits theaters July 29.

We caught up with Walsh to talk about working with Damon, everything that goes into getting an actor in shape, fitness culture in Hollywood and more.

On working with Damon

Walsh's top priority is ensuring that actors are strong and healthy. Their physical appearance, he says, is just a side effect of building a solid foundation. Damon had back and shoulder problems when he first came to Walsh, and fixing these issues was paramount. Not only do actors need to look good, their bodies need to be able to endure a strenuous film shoot, which often includes stunt work or wearing heavy equipment for hours on end.

"[Damon] came to me before The Martian in 2014 and he had already decided that he was going to do the three movies: The Martian, The Great Wall and Bourne," Walsh says. "All of them were in succession within like two weeks of each other. He just winded up and crushed them. A couple of months before The Martian started filming in Budapest, we started working out to get in shape for that. He's playing an astronaut, and astronauts are in shape. He wanted to look like he was in shape. We didn't have to get ripped for it. It wasn't anything like that. For me it's about making sure their body is resilient and healthy and pain free, and that they stay that way throughout the filming of the movie. He had to wear a space suit and he had to run and he had to do all these stunts and there were long days. The last thing you need...If he gets hurt, production stops. It's a huge, huge responsibility. You've got the studio looking at you, you've got agents, you've got everybody looking at you and what you're doing with this guy. It's a huge commitment."

On the type of instruction he gets from directors

While strength and health come first, Walsh often gets specific instruction from the film's producers and director about not just what an actor needs to look like, but how they need to feel. Bradley Cooper didn't need to simply be ripped to play Chris Kyle in American Sniper, he needed to be big. He needed to feel like Kyle. For Bourne, as well, Damon's physicality was directly tied to his mindset.

"I think it depends on what they're going to be doing in the movie," says Walsh. "For Bourne, I talked to the producers and Paul Greengrass, the director, who wanted me to know and understand that this is the opening sequence of the movie. We want to set the tone for the rest of the movie and let people know how distraught Jason Bourne was. It sets the tone for the movie. He's really messed up in the head. He's still having these flashbacks. He can't live like that. He's getting older. He's stressed out. You can see the stress on the body. We actually trained that way right before the beginning of that scene. We trained him until he was pretty exhausted. We did hill runs. We ran these enormous hills, and it just sucked. It was almost to the point of overtraining. You could see it. He was exhausted. That was a good thing because that's what Paul wanted, that he was just screwed up in the head."

On how much time he needs

"Three months is a great number to really make big changes in," says Walsh. "We had two and a half months with Emily Blunt for Edge of Tomorrow. She was a badass. I got her so damn strong for that movie, and so conditioned. She had to wear a 60- or 75-pounds body suit around for hours and hours and hours. I had to get her so strong and resilient so that that body suit doesn't break you down over the day."

On PED use in Hollywood

Not everyone uses a trainer like Walsh.

"I think it's pretty rampant, to be honest with you," says Walsh. "I think Vanity Fair did a big article on it, about HGH in Hollywood, and steroid use, and stuff like that. I don't blame [the actors] sometimes. These guys get a part and are told that in four or eight weeks that they have to look a certain way. Sometimes the expectation is far beyond the possibility unless they take something like that. I just don't want to be a part of it."

"It is a problem here in Hollywood," he continues. "It's unfortunate because it sends the wrong message to young people, who look at [an actor] and think, 'Wow, this guy is really in shape.' It's kind of a lie. Matt Damon in Bourne was really in that shape. He worked his ass off, got really strong, and then we conditioned and tweaked the diet to take that extra layer of fat off so that he looks jacked."

If trainers aren't suggesting stars use PEDs to get ripped in record time, they might be putting them through a strenuous body-building program that might get them jacked, but ultimately isn't very healthy and can lead to injury. "I think there is this old-school, bodybuilding mentality," says Walsh. "Bodybuilding is useful. There's definitely some application for it. But it's not the go-to every time. One formula doesn't work for everybody."

On working with women

Walsh also makes sure women like Emma Stone, Emily Blunt and Jennifer Aniston are in shape for upcoming films in which they may or may not be showing some skin. As is the case with the men he works with, Walsh value health and well-being above any aesthetic result. Not only to actors have to look good, they have to feel good, and their body needs to be strong enough to withstand the rigors of a grueling shoot.

"It's too bad, but it's confusing for women," he says. "They're told that this is the way to work out and this is the way to diet and this and this and this. There are so many damn people in this market out here. There are so many outlets as far as how to train and who to train with, because the pressure is so much more. What I do best with these girls is to get them over these mentalities that lifting weights and during certain things that [men] do are not going to make you bigger or bulky or anything like that. There is nothing like being strong. To be able to move correctly and move with confidence and be strong in your movement, it's a confidence and a wave of life that you're not going to gain in a lot of these other realms. [The other stuff is] kind of fluff to me.

"I was working with Alison Brie [this morning]. She's comedic, but she's also a little badass. She gets up there and can pop out five or six full-on pull-ups, which is kind of unheard-of for women. You look at her, she looks strong, she looks good and she feels great. She exudes that confidence. She's happy with who she is and how her body looks. It's too bad that the pressure has pushed [women in a different direction]. So I just want to empower women however I can. You might start with a lot of body weight stuff, teaching them how to move. But it's an education, and when you finally put weights in their hands, it makes sense."

On how fitness culture has evolved

Fitness is bigger than ever, especially in Hollywood, where looks can make or break careers.

"The fitness industry is growing," says Walsh. "I think the interest in people getting healthy and working out has grown dramatically, especially in the last six to 10 years. People are much more aware about it. There are certain trends that are happening. People are getting interested. It's becoming much more integrated into our culture. You've got to keep up with the times and stay healthy. It's a narcissistic kind of world out here, so you've got to pay attention to it. It's changed, though. It's not as much about what's in the mirror as it is about staying healthy, which is a good thing."