Sydney Sweeney spent three months training for her role in Christy

In a Q+A at Hamptons International Film Festival, Sydney recalled: "Every morning I would weight train for an hour, then I would go and do boxing for three hours, and then another hour of weight training at night. I did that every single day for three months, with a nutritionist, so eating a bunch of food and protein shakes and supplements. I, in that span of time, gained, like, 35lbs."

Sydney explained that Christy was "very involved" during the making of the film and felt that her presence was invaluable as she was keen to perfect her portrayal of the boxer.

The Echo Valley actress said: "She kind of became my best friend. Having her with us was a really powerful experience.

"I've never had the person who I'm portraying watch me. You're never really sure what they're thinking. But as it went on, I just wanted her there the entire time because I was able to study her and listen to her and have her thoughts and opinions there whenever I needed."

Sweeney added: "I try to lose myself completely for a role, so I try not to carry any of my own stuff into their life. But I honestly learned so much from Christy that I applied to myself."

The Euphoria star previously stated that she was determined not to use a stunt double for any of the boxing scenes featured in the movie.

Sydney said: "Every single fight you see, we are actually punching each other. We are going full force.

"I always believed that you would not be able to make it feel real if it's a stunt double or if it's faking the hits."

Christy Martin became the most well-known female boxer in the US during the 1990s and was also the victim of an attempted murder by her abusive husband James Martin in 2010 - with Sweeney shocked that her story hasn't been told on a "global" level.

She told Vanity Fair: "I was blown away that her story wasn't more known on a universal, global level because it's just one of the most harrowing and inspiring women that I've ever met in my entire life."