Academy Award winning actress Emma Stone put in some serious work in preparing for her role as tennis legend Billie Jean King in Battle of the Sexes, which premieres today. The film follows the most watched televised sporting event of all time, the 1973 tennis match between women's tennis star, Billie Jean King, and former Wimbledon champion, and then 55-year-old, Bobby Riggs. Before playing King on the court, however, Stone hit the gym with trainer Jason Walsh to build an unbelievable amount of muscle—15 pounds—for the role. Here, the Rise Nation founder breaks down the workouts and shakes responsible for achieving the impossible.

Generally speaking, what goes in to being a Hollywood trainer?

I’m a trainer in Hollywood, but I don’t like to be called a Hollywood trainer because I train a lot of people, not just actors. The only other difference, honestly, is that these guys are going to be looked at a viewed by millions of people. They get a lot of feedback that most people would never have to hear. That being said, we get to prep them and get them completely ready, both physically and mentally, for their roles. Most people don't think about the mental aspect. What I really try to do is get them to feel 100 percent confident in their own skin. I also make them resilient. Emily Blunt for Edge of Tomorrow had to wear a 70-pound exoskeleton all day long, which is a lot. They’re actors not stunt men. Matt Damon does a lot of his own stunts, like the fighting. If their body isn’t sound, then these guys are going to get hurt. For the most part, the actors that I work with are major assets worth a lot of money. When they get hurt, production stops, and let me tell you, that’s a lot of money. I’m very proud of my clients. When I first meet them, I’m like, “Listen. This is not going to be easy." We don’t cheat. We don’t use any enhancement drugs or anything like that. If I can’t get it done naturally, I just turn the job down. I don’t believe in that stuff and nor would I ever partake. So I tell them how long and arduous the process is going to be and if they’re up for it, I put them through it.

And with Emma Stone, she really had to work for the role of tennis player Billie Jean King.

She had to really had to work for the role. The good news was that she’d been training with me for a while, so she knew to a certain extent what she was getting herself into. Her body was very functional and she was very balanced and she had a great foundation to start from because I had trained her for La La Land. For Battle of the Sexes, she needed to be resilient and strong and mobile. Emma was very petite and so we needed to put some size on her, we needed to get her to into athletic shape in general. As far as the tennis focus, we did do a lot of shoulder strengthening; rotational, anti-rotational work, things like that because of the motion of and how the body works for her. Her legs are strong. Her butt was extremely strong which stabilizes the hips and helps with everything else. If you’re going to move like a tennis player you’ve got to have that foundation where you can move like that. What we did is we gave her the ability to do that without breaking down and getting injured.

Did you follow a functional fitness approach?

It kind of goes hand in hand. I think it’s an overused term. Outside of body building, most training is functional, or it should be. If it incorporates a lot of primitive movements like squatting, lunging, carrying, pushing, and pulling--it should be functional if it’s done correctly.

Did the training involve a lot of strength work?

Yes. Here’s the thing, most people equate strength training with heavy weights automatically with bulky muscle. There are ways of formulating the training that we can exploit strength training for methods beyond bulking up. For Emma, we really had to tinker with her calorie intake and diet, along with upping the amount of training that she did for Battle of the Sexes. When she trained for La La Land, we worked out three to four days a week. With Battle of the Sexes we did five days a week and sometimes two times a day. Between the increased frequency of workouts and addition of calories that were good, healthy calories, she stayed pretty lean.

Is it harder for women to put on muscle than it is for men?

Yes, it is absolutely harder for women to put on massive amounts of muscle. When women come in and I tell them that we will most likely have them doing pull ups they say, “There’s just absolutely no way.” And you know what, they actually have a really good point. Women’s bodies aren’t really made to do certain things. You don’t have a lot of upper body strength, but can we exploit and train the body to get to the point and teach it that that’s what we want it to do? Absolutely. You know what? Most of the women I train are doing exercises and things that are far beyond their mental reach as far as what they thought they can do. And that’s when it becomes really fun.

And so putting on 15 pounds of muscle, especially for someone as lean and fit as Emma, is a big deal.

It’s a huge deal. It’s a lot of consistency and properly stressing the body in the right way and creating an environment for chance. When I say creating an environment it’s, you know, putting enough of the right kind of calories in the body to give the body the ability to adapt properly. Hydration and soft tissue work is also key. You want to make sure that the wheels don’t fall off during the training. And that’s always a risk.

What types of quality calories that you reach for in building muscle?

We stay away from a lot of the simple sugar that she’s not going to need. We actually put quite a bit of good fat into the diet which is calorie laden and make sure that she's getting a ranges of proteins and micronutrients as well. It’s so multilayered as far as diet goes. It’s never just one simple thing. What really helped us with Emma, was having some really good shakes because when you’re under that much stress, the kind of stress from lifting and pushing and pulling hundreds of pounds, the body’s metabolism naturally is going to skyrocket. People are like, “She’s insatiable. She can’t get enough food and she’s hungry all the time. She wakes up starving.” And that’s natural. She’s naturally going to want to increase the calorie intake, but sometimes it just becomes so mentally tough to actually eat a lot.

What I do is have people drink calories. And that means that we add in a couple shakes, and in those shakes it’s really easy for us to add a really high-quality protein that athletes use. It’s super clean and tastes really good. It’s called Metabolic Drive. If it doesn’t taste good most people are not going to drink it. It’s a really good tasting protein and then on top of that we put Udo’s oil, which is a super clean. Every cell in the body is made of lipids, or fats, so you’ve got to give the body the ability to recover. The brain is made out of fat. Fats are a good thing.

How long are these twice a day workouts?

On Monday, Tuesday, Thursday, Friday we would do two a days, and it would start with an hour in the morning with our complex heavy lifting. And then she’d come back in the afternoon and we would do much more of the isolation stuff where we would focus a little bit more on the aesthetic stuff. We would also do some conditioning, but I didn’t want her to necessarily do a ton of conditioning because she got a lot of that just from the lifting she was doing.

For guys who are reading this, could they use everything that you employed for Emma to build muscle as well?

Oh, my. Trust me. Emma was impressive in the gym, when new guys would come in and start training with us, they would literally just be in awe of the stuff that we were doing. Jaw dropped. She could run circles around them.

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