Actress Brie Larson gained 15 pounds of muscle by lifting extreme weights for her acclaimed role in Room.

The 26-year-old has already won a Best Actress Golden Globe, Critics Choice and Screen Actors Guild Award, and she's the favorite to pick up the Academy Award in the same category later this month too.

The critical acclaim is certainly valid, as Brie transformed her body inside and out to play a young mother trapped in a small room for seven years, building up her physical strength as well as her mental resilience.

"I learned so much, but I think strength became the biggest one," she told People magazine. "There are many ways that I learned about the strength. One was gaining 15 pounds of muscle, and I was able to lift things that I never felt in my life I would be able to do."

Brie wanted a wiry but muscular look for her character, Joy, and achieving it involved getting up extra early to train before a long day of filming.

"That became such a huge part of the routine of making this movie," she explained. "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."

Her determined attitude towards her training regime also translated mentally to how she approached her character and the script.

"It was an emotional marathon to make this," Brie admitted. "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."