Sydney Sweeney gained 30 pounds to play boxer Christy Martin. She also went through about three and a half months of training, which involved weight-training in the morning for an hour, kickboxing midday for about two hours, and weight-training again at night for an hour. She said in an interview with W magazine, "My body was completely different. I didn't fit in any of my clothes. I'm usually a size 23 in jeans, and I was wearing a size 27. My b**bs got bigger. And my [bottom] got huge. It was crazy! I was like, Oh my god. But it was amazing. I was so strong, like crazy strong."
According to David Michôd, the director and cowriter of the film, "Christy Martin put female boxing on the map in the mid-'90s. She was the first woman fighter on the cover of Sports Illustrated and fought on the Mike Tyson undercard. Her husband was also her trainer, and then, after her success, he tried to murder her. Our film is a wild mix of inspiring underdog sports-world story and personal saga."
In 2018, Amy Schumer announced her interest in playing Martin in a planned biopic but the project never developed.