Which brings us back to Room. The role required Larson to scream, cry, and yell — not just at her captor, but also at young Jack and the family with whom she is reunited. (That's not a spoiler! It’s in the trailer to assure you that the movie isn't a total downer.) To embody Ma’s wiry strength, Larson trained and dieted intensely for the entire 49-day shoot, a transformative experience that went beyond the physical. “It was the first time in my life that I had muscle, so I had this completely different relationship to how I felt in my body,” she says. “I felt so much more confident and comfortable. To get to the end of that shoot knowing that I could handle the emotional weight and I could hold 75 or 100 pounds over my head, something that I never thought I could do… You really don’t know what you can handle until you start to push yourself.”

Her colleagues sure have taken notice. "I had seen her in Short Term 12 and was completely blown away," says Joan Allen, who plays Larson's mother in Room. "There's just no BS, no flash, no putting on. And I think it's great to see a woman who's not glamorised but is high-functioning in a really difficult circumstance."

To recover from the experience of living as Ma, after filming wrapped, Larson headed directly to Hawaii for some R & R. “I hadn’t been in the sun for so long,” she says. “Getting some vitamin D and eating rice — I hadn’t had a carb in so long — I could feel myself coming back.” Then it was time to return to her real home in Los Angeles, a readjustment that she related to differently after acting out Ma’s more wrenching homecoming. She's philosophical about how she approaches this part of her job. “When you spend so much time living life as another person, you get back home and you get this opportunity to sort through it,” she says. “Some of it is kind of dark and not something I need to hold on to. But some of it is interesting, and I can revise an old draft of myself to incorporate those things.”