Actress Sydney Sweeney has revealed the gruelling transformation she underwent in order to play a boxing icon in her latest film.

The star, 28, called the role of Christy Martin a "once-in-a-lifetime opportunity" and "a beautiful gift". But stepping into Christy's boxing boots meant months of training, a dramatic weight gain and "19-hour marathon" days.

Sweeney detailed: "I had about two and a half to three months to prepare. I worked with a weight trainer, a nutritionist, and a boxing coach, training every single day.

"In the mornings, I did an hour of weight training followed by two to three hours of boxing. At night, I did another hour of weight training and consumed three to four protein shakes a day, along with endless calories and supplements, constantly working to transform my body.

"I put on 35 pounds for Christy and then had to maintain that while filming, which was honestly the hardest part. Before, preparing was all I did, but during filming, you have to keep it up while performing, and that was a whole new challenge.

"And once filming started, I had to maintain the training, or I'd lose the muscle and mass I'd gained. So it wasn't just 12 hours of filming, I'd wake up early for an hour of weight training before work, then film for 12 hours, followed by fight training and more weight training.

"My days turned into 18 to 19 hour marathons, every single day."

'She's absolutely inspiring'

But the Euphoria star also reflected on the "honour" she had of playing Christy, who withstood more than just the physical trauma of fighting in the ring. Known as 'the Coal Miner's Daughter', Christy Martin became one of the most accomplished female boxers in the United States.

Now 57, Christy endured endured domestic violence from her manager and husband, Jim Martin, and Sweeney's new biopic - directed and co-written by Australian filmmaker David Michôd, documents her ascent to becoming America's most prominent female boxer in the 1990s and her husband-turned-coach's 2010 attempted murder of her.

Christy, starring Sweeney in the title role and Ben Foster as her husband Jim, seeks to highlight patterns of abuse and amplify Martin's message of resilience. Sweeney hopes the film will provide hope to survivors of domestic violence.

She added: "I learned so much about myself, and I have the honour and the blessing of getting to have Christy in my life forever. She's absolutely inspiring.

"Christy is one of those roles that comes along once in a lifetime, the kind an actor dreams of. I've always wanted a character I could lose myself in, one that would truly challenge me, and Christy was all of those things. She was a beautiful gift, and one of the most inspiring people I've ever met.

"I keep saying that I want to be 'Christy strong,' and I really hope it inspires others, that they know they're not alone and can find the strength within. I hope Christy opens doors for other people."

Breaking the cycle

Michôd, who penned the screenplay alongside his real-life partner Mirrah Foulkes, reveals he recognised Sweeney as the perfect choice for Christy's role within moments of their first conversation. "I knew within five minutes of Zooming with her during the first script read that she wanted this, she was hungry for it," explains Michôd.

"This character needed an actor with that drive. I didn't want to have to persuade anyone; I wanted someone ready to chase it down, to hunt it down.

"That was Syd. That hunger alone made me think she was the right person, aside from her other qualities, her incredible acting skills, and her fight training. It was just the hunger."

Co-writer Mirrah Foulkes expressed her wish that the production will heighten understanding of abusive dynamics and could help stop similar situations occurring. She explained: "It's my hope that the more we depict these patterns of abuse on screen and in media, the more familiar people become with them, and the more likely we are to intervene before it goes as far as it did in Christy's life.

"I hope young women see this film, I hope young men see this film, I hope all kinds of people see it. The more familiar we become with the steps these relationships often take, the more likely we are to start breaking the cycle."

Christy arrives in UK and Irish cinemas from Friday, 28 November.