How much did Sydney Sweeney lean on Christy Martin for the movie "Christy"? The actress says the former world champion changed "everything" for her.
In "Christy," Jim Martin (Ben Foster, left) plays the trainer and abusive husband of world champion boxer Christy Martin (Sydney Sweeney).

Sydney Sweeney admits she "has horrible stage fright." The details aren't for the squeamish.

"I usually throw up before having to go on stage and do a talk show or anything like that," Sweeney, 28, tells USA TODAY. "I get really nervous."

The star of the new sports biopic "Christy" (in theaters Nov. 7) is on Zoom from Los Angeles to promote the movie alongside her costar Ben Foster, who joins from Nashville. And Foster is learning a thing or two about his colleague as she elaborates.

Sweeney recalls her anxiety started when she moved to a new middle school in Washington state. "I remember my mom was driving me to my first day and the moment I saw the school's signage outside the gate, I threw up in my gym bag because I was so nervous."

"I've heard Pavarotti used to do the same thing," Foster, 45, offers. "Fear is just weakness leaving the body."

While it's unknown whether or not the tenor actually tossed his cookies before performances, Pavarotti did speak of anxiety and stage fright in multiple interviews.

In "Christy," Sweeney portrays female boxer Christy Martin, who overcame sexism and abuse to become a world champion before her husband/trainer Jim (Foster) tried to murder her. The two actors underwent physical transformations and emotional discoveries. Through different routes, they also gained the trust and friendship of Martin.

Sydney Sweeney, Ben Foster packed on the pounds in different ways for 'Christy'

To play the boxer, Sweeney trained for two-and-a-half months prior to production and gained 35 pounds. At Toronto Film Festival, the actress credited her weight gain in part to "a lot of Chick-fil-A, a lot of Smucker's, a lot of milkshakes, a lot of protein shakes."

"I had seven weeks in between wrapping 'Christy' and starting 'The Housemaid' and 'Euphoria,' " the actress tells USA TODAY, referencing the psychological thriller she stars in alongside Amanda Seyfried and her HBO series. "So I definitely was on a tight timeline, but it was a lot harder to gain the weight than it was to lose it. Then it was just being really strict with myself and making sure that I was doing a lot of cardio and eating very clean."

Sweeney says she lived with "Christy" longer because of her transformation, but feels it's key "separate myself from my characters" to protect her mental health. Some roles have taken her to "some treacherous and just very crazy emotional places," she says, "So it's important for me to be able to jump out of it."

As for Foster's transformation into Martin's abusive husband, the actor deadpans that he "just ate a lot of burritos and did less cardio." It worked so well that Martin, who was on the set for the film, thought that the actor's appearance and portrayal of her ex was perhaps too spot-on.

Foster recalls a run-in with Martin on the set. They were passing each other in the hallway when she "started cursing, turned around and walked the opposite direction." Foster thought he did something to make the boxer angry and worried he might get fired from the film.

"It was communicated to me that (Martin) just said, 'I'm not talking to Jim, that's Jim,' " Foster says. "It's a strange backwards compliment."

Martin didn't speak to Foster until production wrapped. They've since become friends. As for the boxer's relationship with Sweeney, the two became very close.

"She has such an amazing strength within herself," Sweeney says of Martin. It affected the actress both on and off the set. "I learned to speak up for myself a little louder and walk around with my head held high."

That's something that hasn't always come easy for Sweeney. She characterizes the town she grew up in as a place "where no one leaves and they all kind of do the same thing." That thing isn't pursuing a career in Hollywood, and Sweeney's aspirations made things "really hard" growing up. (Like Sweeney, Martin is also from a small town and nicknamed "the Coal Miner's Daughter" as she hails from West Virginia.)

"I got a lot of hate and a lot of bullies," Sweeney says. However her past experiences haven't ruined the actress' trips back home. Sweeney hopes she "can inspire others to choose their own path and to make change."

She hopes that audiences find the same inspiration she did in this movie.

"I always want to encourage other people to be 'Christy strong,' " she says. "I hope that when they watch this movie, they find that within themselves."

Contributing: Brian Truitt

This article originally appeared on USA TODAY: 'Christy' star Sydney Sweeney reveals 'horrible stage fright': 'I usually throw up'

Reporting by Ralphie Aversa, USA TODAY / USA TODAY

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