The SAG and Golden Globe Award winner tells PEOPLE that she thinned down and started weight lifting for her role as a kidnapped woman
Brie Larson
Credit: Kevork Djansezian/Getty

To play a kidnapped mother who has lived in one small room for seven years in Room, Brie Larson transformed her body – from the outside and in.

Larson, 26, tells PEOPLE that she worked with an “incredible” trainer to add 15 lbs. of muscle to her frame, an experience that changed her mindset.

“I learned so much, but I think strength became the biggest one,” Larson tells PEOPLE after winning the Screen Actors Guild Award for best actress. “There are many ways that I learned about the strength. One was gaining 15 lbs. of muscle, and I was able to lift things that I never felt in my life I would be able to do.”

To get the wirey, but muscular look Larson wanted for her character, she would get up early to strength train before filming.

“That became such a huge part of the routine of making this movie,” Larson says. “Every other day, I worked with this incredible trainer, and she would have me lift extreme weights over my head.”

“Just having that, being able to deadlift before I went to work every morning, gave me this mental change that I never had before.”

And that physical strength and determination translated mentally as she dove into her difficult role as a mother trying to give her son a happy life despite their tragic circumstances.

“It was an emotional marathon to make this,” Larson recalls. “There are moments when you think, I just don’t have anymore to give. You realize that there’s more, and that life keeps going, and that it is hard. That’s okay.”

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Her hard work is paying off, with wins for her critically lauded performance at the Golden Globes, Critics’ Choice Awards, and SAG Awards. All signs are now pointing to Larson taking home her first Oscar during the ceremony on Feb. 28, though she’s not letting the buzz get to her.

“The interesting thing about our brains is they can’t figure things out in the moment,” Larson previously told PEOPLE. “So, I’ve relieved myself of trying to understand it in the moment.”

“When I’m sitting in bed – that Brie I know,” Larson says. “But I don’t know the Brie in sky-high heels on a carpet with a bunch of people screaming at me. I wonder what she’s like. Either way, I’ve got an awesome therapist and an awesome family, and two really cool dogs, so I’m good.”