Sydney Sweeney gained 35 pounds, suffered concussion filming new movie

Sydney Sweeney packs a punch both on and off the screen.

The "Euphoria" star shared how she transformed her body - and her grandmother's backyard - to prepare for her new movie, "Christy."

Sweeney portrays boxing trailblazer Christy Martin in the upcoming biopic, and said she gained 35 pounds to play the role, training like a professional fighter in her home state of Idaho.

"It was so much fun though, because I grew up in Idaho with my family, and my grandma's my neighbor, and I turned her shed into my 'Rocky' gym," Sweeney told "Good Morning America." "I put flooring in and boxing bags, and I brought my trainers up there, and I had a boxing coach and a weight trainer. And every single day I would weight train in the morning and night, then I'd box for like two or three hours a day."

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Sweeney said she fueled her transformation with "protein shake after protein shake," and the intense regimen even led to some battle scars on set.

"We were knocking each other out, we were getting bloody noses, I got a concussion," she admitted. "We were actually fighting."

The actress said her connection to the story went deeper than the physical challenge.

"[Christy] was a professional boxer who came up during a time when women in the sport were unpopular," Sweeney said. "She was a trailblazer for women boxers. She ends up marrying her trainer … who unfortunately manipulates her, and she becomes a victim of domestic violence. And when she tries to leave him for the love of her life, who was a woman, he shoots and stops her."

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The true story of Martin - known as "The Coal Miner's Daughter" - captivated Sweeney, who also serves as a producer on the film.

"I remember when I got on the first Zoom with the director, I said, 'I don't care, I want to produce this, I just want to be a part of telling her story,' because I think that, like Christy says, she wants it to save lives, and I want to be a part of telling that and hopefully opening doors for other people."

She added, "It's unbelievable. I wanted to lose myself for this."

And to make the experience even more surreal, the real Christy Martin was on set watching Sweeney step into her gloves.

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"It was so fun having her there, especially when we were doing the fighting scenes," Sweeney said. "I could hear her at the bottom of the ring going, 'Hit her with a left hook!' Like, 'You know what I'm going to do, Christy!'"

The "Anyone But You" actress turned heads Tuesday as she stepped out in Manhattan, wearing a short leather trench jacket - and nothing underneath.

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The star's eye-catching ensemble featured a nude leather Sportmax trench coat styled as a dress, matching knee-high boots and a Miu Miu bag. She completed the bold look with tinted sunglasses.

Sweeney, who rose to fame for her role in "Euphoria," portrayed Cassie Howard, a high school student known for her complex relationships and struggles with self-worth and validation.

She even teased what's next for the upcoming third season of "Euphoria," saying, "The only thing I can say is it's about God. Some people are going to find God, some are going to need God."