EXCLUSIVE: Emma Stone on Why Her Physical Transformation for 'Battle of the Sexes' Became 'Addictive'

Emma Stone takes winning seriously.

The 28-year-old actress did all she could to imitate Billie Jean King in her new film, Battle of the Sexes -- and it definitely paid off. 

"It was amazing," King, who served as a consultant on the film, told ET's Cameron Mathison at the junket on Saturday. "She captured my voice. I was like, 'How could anyone do that so well?'"

Stone also tried her best to capture King's physicality, gaining 15 pounds of muscle for her role as the six-time Wimbledon Singles champ. 

"A lot of weightlifting, a lot of protein shakes," Stone confessed of how she bulked up. "I never did it before and I loved it." 

"For the first three weeks, you hate your life, and I know a lot of people hate exercising. I did. And then after three weeks when you're, 'I can lift this? I can do this?' It just becomes addictive," she explained. "It's just really incredible to feel strong. It feels so good."

Battle of the Sexes depicts the epic 1973 match between King and then 55-year-old Bobby Riggs, who is played by Steve Carell. And while the actor went through a physical transformation of his own, it's his cosmetic changes people can't get over. 

"I read a review that talked about how bad my wig was, and it wasn't a wig," he admitted. "It was my hair." 

The last time Carell worked with Stone, he played her father in 2011's Crazy Stupid Love. 

"She's played my daughter and now she's playing my adversary in this. In this next one, I want her to play my mother," he joked. "She could do it. She's that good."

"She's, like, the most delightful person in the world," he added. 

Battle of the Sexes hits theaters on Friday. See more on the film in the video below.