, Oct. 6 -- Actress Sydney Sweeney revealed the rigorous preparation she underwent to play boxing champion Christy Martin in the upcoming biopic Christy.

Speaking at the Hamptons International Film Festival, the 28-year-old said she followed a punishing routine: "Every morning I would weight train for an hour, then do boxing for three hours, and another hour of weight training at night. I did this daily for three months with a nutritionist, eating lots of food, protein shakes, and supplements. I gained around 35 lbs (15.88 kg) during this time."

Sweeney added that Martin was deeply involved in the filmmaking process, offering guidance and feedback. "She became like my best friend. Having her there was powerful, as I could study her mann...