Sydney Sweeney is opening up about her major transformation to portray Christy Martin. The actress, 27, revealed her starring role as the pioneering boxer in October 2024, sharing photos of her dramatic transformation into Martin on social media. Now, in an interview with W Magazine published June 9, Sweeney is shedding light on the process behind her bold new look. Sweeney gained over 30 pounds to mirror Martin's toned figure, she told the outlet. “I loved it. I came onboard to play Christy, and I had about three and a half months of training," she said. "I started eating. I weight-trained in the morning for an hour, kickboxed midday for about two hours, and then weight-trained again at night for an hour.” She started seeing changes soon after. She couldn't fit in many of her clothes, went from a size 23 to size 27 in jeans, and saw a "crazy" increase in the size of her bust and butt, she said. “My body was completely different," she said, adding, "But it was amazing: I was so strong, like crazy strong.” Sweeney "trained her butt off to play the part" in the biopic, said director and co-writer David Michôd, and she was excited every day to get into character. Just seven weeks after filming for the biopic had concluded, Sweeney was on set for another project — the highly anticipated third season of Euphoria, where she plays Cassie. When Michôd FaceTimed her on the show's set, he noted her transformation from Martin's physique was just as drastic as the original change. "I was stunned by the change in her appearance. I had only seen her as an adorable, tough moppet with late-’80s hair, ready to box," he told W Magazine. "Sydney was happiest when she was in the ring punching and being punched. It was shocking to see her being so glamorous.” Never miss a story — sign up for PEOPLE's free daily newsletter to stay up-to-date on the best of what PEOPLE has to offer​​, from celebrity news to compelling human interest stories. The rigorous physical demands of the role weren't entirely new to her. Sweeney's mother encouraged her to "fall in love with as many things as possible" as a child, so the actress sought out sports and outdoor activities, like skiing, wakeboarding, soccer and softball. She even dabbled in grappling (like wrestling, she told W) and kickboxing. Sweeney never let her acting career stand in the way of her love for adventurous activities, she said. "I would have bruises all over me," she said. "I loved it! My mom always wanted to make sure that acting wasn’t the only thing in my life.”