One day I'm looking at John Krasinski's delts in Men's Health, and the next I'm looking at the guy behind the star's 16-week transformation from "50-quesadillas-in-one-sitting fat" to ripped Navy SEAL in 13 Hours: The Secret Soldiers of Benghazi. Jason Walsh is in Las Vegas for another action film, the latest installation of the Bourne saga, as Matt Damon needs daily maintenance in the gym to keep him in peak form on set. And if you caught Bradley Cooper's recent Oscar-nominated turn in American Sniper, his beefed-up physique was also orchestrated by Walsh. So much for milk doing a body good.

You might expect someone who's spent his life studying and manifesting muscle to be kind of a meathead. Or that training celebrities on exotic production sites and at his boutique gym in West Hollywood might warp his sense of reality. But Walsh comes across as a thoughtful, down-to-earth guy who worked really hard and feels he got really lucky. He loves wine and his rescued pit bulls and Nine Inch Nails. His wrist is tattooed with the name Hazel, his grandmother's, and he talks a lot about the influence she and his uncle had while he was growing up poor with a teenage mom. "My grandmother would talk to me about rising above. You’ve got to rise up in order to better your life."

Walsh wonders if subconsciously, that's why he named his training gym Rise Movement, and his offshoot group-fitness program Rise Nation. The year-old concept involves high-impact routines on VersaClimbers, with pump-up theatrics like a pitch-black studio hit with dynamic, almost nightclub-ish lighting synced with sick music. While it's in its infancy, Rise Nation is a unique model Walsh hopes to bring to big cities like Vegas and rural towns alike. "It’s not always about profits to me. I want people to have outlets I didn’t have. I would have been excited to have something like this in my hometown of Springfield, Missouri. There were just a few gyms, and they weren't that great. I mean, they look great now when I go back home, but when I was there you’d be lucky to get a staph infection," he says with a chuckle.

On a peaceful promenade inside Aria (where Bourne is shooting), we sat by a digital fire and talked about Walsh's journey. Not to mention the undying badassery of Jane Fonda.

How far back does your love of fitness go? I’ve been a gym rat since I was a kid. That was my solace and my meditation. It’s changed forms throughout my life. It was applied to sports; it made me a better athlete, so I was like, well sh*t, this is worth it. I’m gonna be better than my peers or at least contribute more to the team because I’m a better athlete. And then it was, the girls liked it. (laughs) It changes forms and you grow up and you use it for different reasons. Now it’s like, I want to be pain-free, and I want to age better. I just turned 40, and I work with a lot of middle-aged adults. So my philosophy and my story of what I’m trying to do for people has changed.

I've never spent much time at the gym, but I love being active outside. I love recreational sports. If I could get people to go out and do that more I would advocate that—but you’ve gotta do the work in the gym. You’ve gotta keep the alignment in the car and the oil changed so it runs smoothly and it performs better. And that’s where strength and conditioning is at today, and it’s fantastic, it’s beautiful. Because we have the knowledge and the science now that only pro athletes used to have access to, to keep them healthy. It’s all about numbers and usually about money, and those athletes are worth a helluva lot more money and can make the team a helluva lot more money when they’re on the field, so how do we do that? It’s injury prevention, and that comes from the science of training now.

How did you learn the science of training? I was self-taught. I grew up just getting into the gym and looking at this person or that person and seeing what I wanted from them and mimicking them and learning from them, asking them questions, trial and error. I had that mentality. I was very hungry for knowledge and tried a lot of applications and tried to figure out what worked. I always had that stature in the gym of the guy who knew what he was doing, and I thought I knew what I was doing and I would train people. And then I got into physiology and kinesiology and all of that in college, and one of my good friends who was an athlete recommended me going and talking to Greg Gatz, who became my first mentor. He’s the head strength-conditioning coach for the Olympic sports at [the University of North Carolina at Chapel Hill]. ... I didn’t know sh*t. More or less I knew bodybuilding-type stuff, and when you get into more of the specificity of the art and the science of training, it blew my mind. Here I was thinking I know all this stuff, and it was a whole other world, a whole other level. It invigorated my interest in learning more.

You went from interning with Gatz to being an assistant strength coach at Carolina, but you quit that job to work, unpaid, under Denver Broncos head strength coach Luke Richesson. I took what I thought was all the knowledge in the world—I had it under control; I understood programming and different types of lifting and application—and I worked with this guy and, again, here I was humbled. ... I mean, the amount of knowledge this guy has, it’s on a whole other plane. I stayed and I absorbed and understood that and really took a lot from that.

You ended up moving to California to open your own outfit. Why there? Ten years ago when I moved out there, there were a lot of trainers but there wasn’t a lot going on out there, and I saw an opportunity to possibly make a difference. ... A lot of it just comes down to intuition and gut. I have to say that through this whole thing the guiding light for me has just been trying to do the right thing and be a good person and teach people what I have and be humble and want to learn more and keep that whole continuing education alive. ... And now here I am traveling the world with Matt Damon and other actors doing what I do. It’s like anything, you find your path and there’s forks in the roads, and I end up choosing certain directions on things and making decisions and it kind of guides you through. It’s gut.

Or lack of gut, in the case of John Krasinski. I read that he thought he'd need to bulk up to play a SEAL, but that he learned it's about being lean and strong. We call it efficient. For him, it was knowing who the character was that he played, realizing what these guys are all about—it’s about efficiency and being able to blend in, not necessarily standing out. And so that’s the direction we went with him, whereas Bradley Cooper, the guy he was playing, Chris Kyle, was very different looking, so that was a completely different protocol.

It's nuts that you can mold a human body so exactly, and so rapidly. It’s so beautiful to be able to stress the body in a certain way and watch it adapt, down to a cellular level. It is amazing. If you really stop and think about it, besides birth I don’t think there’s anything that fascinates me more, at least on the human level.

How did you become the go-to guy for getting the action physique? Just dumb luck. Word started spreading about the training that I do. A client would recommend another client, and eventually one of those clients had to do a movie.

Are you ever scared you won't be able to achieve the transformation a studio is looking for? For big projects, I’ve honestly been scared halfway through it and going, it’s not gonna work. It’s so stressful, I can’t even tell you. It wreaks havoc on my life, but it’s still a challenge and it’s what makes it interesting, where you want to quit and pull the parachute, pull the rip chord and get out, but you stick with it and you figure it out and you get through. For me, a lot of that changed when I decided that I was going to train with them, rather than be the guy going, do that, do that and do that. I get in there with them, and I train like them, and they see me hurting, and I’ve been doing this a long time. And they go, "Oh, well if he’s hurting then it’s okay that I’m hurting and it’s normal." I’m not acting. I’m being honest with them. Sometimes it sucks, but a lot of times when you work through I find that not everything worth having comes easy. A lot of the really good stuff takes a lot of dedication, a lot of effort and time and energy. ... I’ve had production companies say, we need this person to get in shape in three weeks for such and such, and you look at them and assess and you just have to be honest. I’ve turned down a lot of business, because I’m like, "I can’t do it. I’m not a magician here. I can get this guy in shape, but I’m gonna need this amount of time." And they may or may not go to another trainer that’s going to give them enhancement drugs, and that’s something that’s highly abused in the world of training. I have no part of that; I’ve never done that stuff, because I like people to realize that there are better methods and long-term results from putting in the time and the effort. To me, that is the whole thing, the quan. (laughs) That’s the greatest gift that I can give to them, that they carry that on for the rest of their life and they understand what it’s about. There’s amazing benefits to training, not just the aesthetic stuff, down to all the physiological elements, mentally, physically, spiritually—it has a huge effect on your life.

Your studio, Rise Movement, what does it look and feel like in pursuit of all that goodness? It’s clean; it’s white; it doesn’t smell like a gym; it doesn’t look like a gym. Of course there’s some equipment I use that would give it away, but it’s just cool. … People feel relaxed and calm when they come in. There’s no slogans on the wall about “be your own champion.” That stuff works for some people. But for me, I don’t need it.

And what about Rise Nation? I started Rise Nation in the attempt to create what I thought was a better, safer, more effective workout for the public.

On the machine from hell. Nobody’s ever been able to break the code of the VersaClimber, it just is categorized under “sucks.” It’s so hard, it’s just the suck machine. It’s stuck in the corner at most gyms. I joke with the guys at VersaClimber that you walk in and see cobwebs on the damn thing because people will give it one shot, and they’ll do it for 30 seconds to a minute, and it kills you, because we’re so far removed from moving correctly that it actually is hard. ... I’m not a machine guy. I don’t like a lot of machines. I like the body to move and stress under movement and weight bear. But these guys from VersaClimber from Orange County, I’m good friends with them, and I’ve always kinda kept in touch and believed in their product. The father who started it back in the ’80s was an engineer and came up with it because he studied the body and the movements and it was a primitive movement. The crawling movement or the climbing motion or the running motion, as we call it, is ingrained in us and is a primitive movement, and I'm talking down to the cellular and neural components that make it up.

So how did you turn the suck machine into an appealing workout? I was on a plane and I was writing stuff out about it, just throwing things down on paper like I usually do on the way to New York, and I had an epiphany, and I was like, No—it can’t be done! A whole class on a VersaClimber? It’s so stupid! There’s no way! And I said to myself, I’m going to figure this out. I’m gonna crack it. When I came back in I put my headphones on and put some of my favorite music on, and I got on it and I started just f*cking around. I think I had old Daft Punk and Nine Inch Nails and Smashing Pumpkins, all the music that really excites me. … I just closed my eyes and listened to the beat, and the forte parts, the crescendos and decrescendos and all this stuff that was going on, and it just happened. And I figured out some of these moves, and I stuck with ’em for a whole song. And I looked up and I had done more feet on the VersaClimber than I had ever done at one go, and I looked down at the ground and there was sweat everywhere. And I was like, I did it. ... It’s turned into something I never dreamed of. I’ve got clients doing close to 6,000 feet. That’s insane. A mile in 30 minutes is insane.

Is it safe? The problem with high-intensity training is fatigue, and your form and the integrity of your training tends to bend with that fatigue, and with that bend can come a break. ... So one of the elements I wanted to go after in group training was safety. ... The VersaClimber is zero impact; it’s reinforcing proper movement patterns, and your oxygen consumption goes up a lot faster and the response that the heart has goes up a lot faster and has a better reaction than any other machine out there, because you’re incorporating the whole body. It all started making sense to me that this is a good thing, we’re gonna help people, we’re gonna fix them down to a basic level, which is movement, proper movement. I want to reinforce proper movement and I want people to get the most out of their training, and if I can get them to get distracted a little bit by the fun of a class, you can get through 30 minutes of something that’s very, very difficult and get a lot out of it.

The fittest I have ever been, I still get my ass kicked by Jane Fonda's old Workout Challenge. The tiny arm circles! She changed the market. She was the CrossFit of her era. Step aerobics, she started all that stuff. Look how that spans through her life. She’s still fit! It’s incredible. I want to be someone like Jane Fonda in a way. I want to be resilient and healthy and have that mentality for the rest of my life. It’s awesome.

Is there anything you can take from foundations like that, as corny and cheesy as some of it looks today? They’re called foundations for a reason. We’re still building off of them. So you can’t discredit it—you can learn from it, and hopefully it morphs and it changes but there’s still elements, there’s still some good roots. Even if it was corny and cheesy, let’s learn from it and take the good elements. Ten years from now it’s all gonna change, but the foundation won’t. It’s the bedrock.

This lighting thing you're doing with Rise Nation is pretty new. Where did that idea come from? I was at a Nine Inch Nails show on one of their last tours, and to me there’s nobody that puts on a production like these guys. … They’re visually just stunning, and it works perfectly and coincides with the music, and it’s just an amazing experience. And the keyword is experience. ... From the very first moment, when the curtain opens and the lights go and the sound and you’re like "Sh*t, this is amazing!" and you get goose bumps. I wanted to capture a little of that feeling and experience. So that’s one of the other big components of Rise Nation, is giving that stimuli to help you get through the workout.

While you're in Vegas for the Bourne filming, what are you doing with Matt Damon? We still train every day, maintenance and keeping him healthy, and making sure he doesn’t get injured. So I’m here for that and I spend time on set, just because I love it and seeing this thing being made. We’re like a family. We all eat together and hang out.

Has being on sets changed the way you see these movies? Yeah, I’ve become a lot more critical. And that sucks! It’s tough because I love movies, and you become a little more critical about things, and also way more amazed about how things look and are edited and turn out.

What about your extensive knowledge of food's effects on the body—has it ruined eating? I don’t like the mentality of cutting yourself off completely from certain things. It’s just about moderation. We probably eat more than we realize, and drinking calories are huge. It’s just math. … My foundation is mostly vegetables, but I also like vegetables—I’m one of those weird kids that liked them. And on the macronutrient level you need the protein for certain reasons. And I used to eat a lot more because I knew what kind of effect it was gonna have on my body. I don’t think like that anymore. It’s too restricting and it takes the fun out of eating. I love food. And I love socializing with friends and going to dinner. I’m not a big drinker but I love wine. I was a bartender all through college.

Do you ever think about making open-source fitness materials, so that anyone could afford a personal trainer? I have thought about that. ... It’d be fun to put something together that’s readily available for the common public, whether that’s a reference that they can have or a DVD or something like that. I just can’t be the cheesy guy. ... I’m just the broccoli. (laughs) I don’t have the cheese. I’m kind of no-frills. I just like hard work and the results that you get from it.

So no rhinestone trucker hats with slogans on them, or big-name sponsorships, even though there's a lot of money to be made. There is a lot of money to be made. … But I don’t wanna be the guy that’s having to bite my tongue and swallow my pride and say something about something I don’t believe in. So it’s tough. I would say I’m probably, out of all the trainers in Hollywood, I make, I’m just not ...

You’re proud to make the least? (laughs) I’m proud to make the least. And don’t get me wrong, we’ve had a lot of success, and there’s a reason we’ve been able to sustain the business that we have there in Hollywood, so it’s not like I’m broke. But I'm trying to stay humble. ... I live in an apartment. I don’t drive a Ferrari. I’m not trying to be someone I’m not, you know? That’s important to me.