Photo Credit, Lead Image: Warner Bros. Pictures, Universal Studios, Universal Pictures

Matt Damon was having fun in 2008. For his upcoming role, he was playing a corporate executive turned FBI whistleblower. The Oscar winner had always been in good shape. He'd already filmed three Jason Bourne movies by that time. But this role was different.

His character was supposed to be chubby and out of shape. The solution: fast food and dark beer. Damon pulled it off and looked the part in The Informant.

The Informant

Photo Credit: Warner Bros. Pictures

But for his next role as an ex-con in the sci-fi movie Elysium, he had to look lean and tough. The movie's director took a headshot of Damon's and pasted it onto a poster of a physique model. Then he took that photo to Jason Walsh, trainer and owner of Rise Movement and Rise Nation in Los Angeles.

Walsh is known for transforming actors, and this time he had his work cut out for him. Not only was Damon in pretty bad shape, he had shoulder and back issues to work around. And Walsh didn't have much time. Damon trained for four hours a day, split into two sessions, and hit the mark.

Matt Damon

Photo Credit: TriStar Pictures

Walsh and Damon teamed up again for The Martian, Jason Bourne, and The Great Wall. Each movie presented its own challenge. In the Martian, Damon had to look fit but also pretty hungry as an astronaut stranded on Mars. In The Great Wall (filmed before but released after Jason Bourne), Damon had to play a monster-fighting mercenary in ancient China.

"The focus for me is always keeping these guys healthy," Walsh said. "It came down to keeping Matt healthy and keeping him from getting injured. It was a very physical role: swinging swords, riding horses, and doing most of it wearing armor."

Great Wall

Photo Credit: Universal Pictures

Walsh often travels with his clients, so much of their training took place in the Gobi desert and had to be improvised. Damon began training to play a tougher, buffer Jason Bourne while still filming The Great Wall. Walsh and Damon would find themselves doing metcon through the streets of Dunhuang in northwestern China.

Back in Los Angeles for only two weeks before Bourne began filming, Walsh put Damon through tough conditioning workouts that included a lot of work on the VersaClimber machines at Rise Nation, a favorite of Walsh's, single-leg work, and countless pull-ups.

"He loves challenges," Walsh told Men's Health. "We got to the point where we were doing 100 pull-ups two or three times a week. We'd see how many sets it takes to get 100. Once you lose some of that excess weight, pull-ups get a lot easier. He was doing over 30 pull-ups per set."

Jason Bourne

Photo Credit: Universal Studios

For Bourne, Damon had to get in the best shape of his life at 45 years old. Walsh reached out to Tim Patterson, founder and CEO of Biotest supplements. Walsh and Patterson have worked together on supplement plans for other actors like Bradley Cooper for his role in American Sniper. Walsh trained Cooper for the role (and even appeared in the movie) and Biotest supplied the supplements.

Damon leaving the gym with Mag-10®

Matt Damon Leaving the Gym with a Bottle of Mag-10 / Splash News

Walsh uses these supplements with his clients:

Plazma™: Workout formula for enhanced growth and recovery. Used before and during the two, 2-hour workouts per day.

Mag-10®: Anabolic pulse recovery formula. Used after training.

Indigo-3G®: Nutrient repartitioning agent. Taken before workouts and evening meals.

Micro-PA®: Muscle growth activator. Taken 1 hour before training.

The Jason Walsh "Secret"

If Walsh has a secret to how he transforms bodies, it's simply this: find weak points and fix them, then build on that stronger foundation. Although he's always under a tight Hollywood deadline, Walsh says it's important to take a step back, access the weak areas of the body, and strengthen them.

For Damon, it wasn't only the old shoulder and back injuries, but core-muscle imbalances, possibly caused by his weight gain for The Informant. Walsh went to work strengthening Damon's back and improving his mobility and flexibility.

And Walsh doesn't just write workouts and tell his A-list clients what to do. He becomes their training partner. "It's really important to get in the trenches with them," Walsh told T Nation. "The guys just really appreciate the camaraderie and the fact that you're in there with them. And it does work. It's the psychological aspect."

Walsh is currently working on another movie with Bradley Cooper. "He's going to play a rock star, so he's got to get super lean and lose a lot of size. Ultimately, he's got to look damn sexy naked. I don't like long distance running or a lot of the stuff we're having him do, but I run with him every week. I hate it! I like to lift weights. I like to be big, around 225. Right now I'm down to 210 and it just pisses me off!" Walsh said, laughing.

Does Walsh ever turn down movie star clients? "Sometimes I'm approached to get an actor ready for a movie and I have to tell them I can't do it. I ask how much time we have, I look at the actor involved, and I have to say, 'Look, there's no way I can do this. I'm going to have to turn this down.' People just have really crazy, unrealistic expectations. I don't deal with performance enhancing drugs or any of that kind of stuff. I've never seen it or dealt with it. So I tell them they're just going to have to find someone else."

Walsh

Photo Credit: Jason Walsh in Locale Magazine, Photographer Erik Isakson

"Travelling the world with these guys is a really cool part of it, but there's a lot of sacrifice. I have two businesses in Los Angeles and I'm opening a third in Cleveland, a fourth in NYC, and fifth in Seattle. It's not that I'm picking to work with Matt Damon over anybody else; it's just an opportunity to work with a guy that I've worked with for years, and I love him to death. It's such an honor to work with someone so great."

The Great Wall opens February 17th.

Related:  The American Sniper Workout