Emma Stone Was 'Ecstatic' to Gain 15 Lbs. of Muscle for Battle of the Sexes, Says Her Trainer

Emma Stone was truly a champ when it came to preparing for her role as tennis legend Billie Jean King in the upcoming film Battle of the Sexes (Sept. 22).

The Oscar-winning actress transformed her body into an athlete’s with the help of her trainer, Jason Walsh, who also got her in shape for La La Land.

Prior to training, “Emma was pretty light and had a very petite, dancer-type body,” Walsh tells PEOPLE. “I wanted to put some weight on her, some muscle on her. We really shifted the diet plan. She started taking in more good, healthy calories.” The intensity of workouts shifted as well, he said. “We started doing training geared toward getting more weight and muscle on her, so it was a little more believable onscreen, and also for her mind and her character.”

And while Stone tells PEOPLE she committed to “about three months” of weight training to play King, the tennis legend stayed in shape while playing professionally.

“It was wild because you weren’t really working out,” Stone says in a double talk interview with King in this week’s PEOPLE. “When you were my age, you were just playing tennis. You looked like that because you were mostly playing tennis.”

King also said her toned physique came from being “naturally muscular,” adding, “I wish I had arms like I did.” To which Stone replied, “I wish I had arms like you did!”

For more from Emma Stone and Billie Jean King on Battle of the Sexes, pick up the latest issue of PEOPLE, on newsstands Friday.

Stone did end up putting on 15 lbs. of lean muscle – and loved it. “She was ecstatic when she got on [the scale],” says the L.A.-based Walsh. “Most girls who get on the scale and gain weight freak out, [but] she had the biggest smile in the world when the weight started to increase.”

And the workouts were intense. “She did up to 300 lbs. on her hip thrusts, which is insane. We’d do farmer walks where she would put heavy weights in her hands — we’d probably do 70-lb. dumbbells in each hand — just walking down the track. She’d do 185-lb. deadlifts and push ups with chains on her back, things that are outside of the scope of what she did before. She really started to enjoy and absorb what I was throwing at her,” says Walsh. “[At first] in her mind there’s disbelief about whether she could do any of this stuff, and when she’d do it she’d scream [with excitement].”

Walsh, who focused on a “full-body training regime” with Stone, also had her attend Rise Nation, his West Hollywood cardio climbing fitness studio. And to make her portrayal of King even more believable, says Walsh, “We would do a lot of tennis moves, like L drills and stuff like that with bungee cords, quick foot box drills to make sure she had her footing and she understood the mind and the feet connection.”

In addition to the heavy training, Walsh’s regime for the star included stretching and soft tissue work like foam rolling, as well as meditation. “Meditation is an aspect of training that no one really talks about that we really implement and suggest. That mind body connection is incredible…it’s just as important as training the body.”

For recovery, Walsh had Stone drink a post-workout shake with Biotest’s Metabolic Drive protein powder. “She loved it,” says Walsh. “It ended up getting to a point where she was like I crave these shakes.”

• With reporting by JODI GUGLIELMI