Sydney Sweeney explains her insane transformation for new film that could be 'defining role'

Sydney Sweeney regularly makes headlines thanks to her stunning looks, and the star is hoping that her transformation for a new movie will win her critical acclaim.

Many actors have made huge changes to their appearance to win awards, from Jared Leto in The Dallas Buyers Club, Charlize Theron in Monster, Christian Bale in The Machinist, and Leonardo DiCaprio in The Revenant.

Sweeney, who recently broke the internet with her American Eagle Jeans campaign, is the latest to undergo a drastic image change for a film.

Fans hope it will be a 'defining role' for her in her career.

The star appears as boxer Christy Martin in the forthcoming biopic, based on the true story of a woman from West Virginia who steps into the ring.

She fights like hell throughout the trailer, but her signature girl-next-door blonde locks have been traded for a short, brunette perm style of the time.

It's clear she has been hitting the gym too and has packed on the muscle for the role playing the International Boxing Hall of Fame star.

Sweeney told Variety that she put on 30 pounds for the role.

“I had a nutritionist work with me as well as a weight trainer and a boxing trainer,” she recalled.

“We upped my calorie intake and I started taking a lot of protein shakes and supplements and eating everything," she added.

We feel quite jealous, to be honest, as Sydney recalled her food intake: "I ate a lot of Smucker's, a lot of PB and J sandwiches, milkshakes, kind of just constantly always eating because we were so active. I was constantly burning it all off at the same time. So keeping it all up was quite a challenge.”

Film critics have called it a 'true-note, game-changing knockout of a performance.'

After the trailer was released online, fans rushed to praise the star for her dedication.

"Sydney Sweeney as a boxer?? If she pulls this off, it might be her career defining role," wrote one.

"Wow - pretty dramatic transformation for Sydney Sweeney. This could be good," added another.

This person added: "Sydney Sweeney acting as a boxer is that last thing I'd have expected her to do."

"Sydney Sweeney as Christy Martin? Let’s go! This one’s gonna pack a punch, can’t wait to see her in the ring," raved another fan.

The biopic tells the real life story of the woman who was seen as a trailblazer in women's boxing, and the hardships she faced with her homophobic mother Joyce (played by Merritt Wever) and her abusive husband and trainer Jim (played by Ben Foster).