Sydney Sweeney at Christy UK premiere at BFI London Film Festival

Sydney Sweeney, who plays former professional boxer Christy Martin, attended the BFI London Film Festival on Friday for the U.K. premiere of Christy, a biopic that traces Martin’s rise in the ring and the abuse she endured outside it. Sweeney, who underwent an intensive physical transformation to portray the fighter, walked the red carpet in a pale pink corseted gown and spoke about the responsibility of telling Martin’s story as the film prepares for a global rollout in November.

Directed and cowritten by David Michod, Christy chronicles roughly two decades of Martin’s life, from an unexpected entry into boxing to becoming one of the most prominent female fighters of the 1990s. Ben Foster plays Jim Martin, the coach turned husband whose control over Martin’s career and personal life the film depicts. The movie also highlights Martin’s later advocacy work; she founded Christy’s Champs, a nonprofit supporting victims of family abuse, and was closely involved with the production, visiting the set during filming.

Sweeney said she felt the weight of that responsibility. "Honestly, it’s a really heavy weight to carry," she said at the premiere. "I feel the importance of her story and...