Room: Why Brie Larson stayed at home for a month

When preparing for her role as a woman held prisoner in a shed in the film Room, Brie Larson decided to isolate herself from the outside world.

"I stayed at home for a month," explains the 26-year-old American actress. "I was excited to see what would come up if I tuned out for a while. I meditate twice a day so I'm very comfortable with silence and the chatter in my head."

Adapted by Emma Donoghue from her own best-selling novel, Room tells the harrowing story of Ma (Larson), a young mother held captive with her five-year-old son Jack (Jacob Tremblay) in 10-by-10-foot space with only a skylight and no windows.

Larson's performance has been tipped as a potential Oscar winner since Room had its first film festival screenings in autumn 2015.

So what did go through her head during her self-imposed exile?

"I reminisced a lot about my past - on certain regrets or moments missed. I figured it was a process that Ma had gone through before Jack came along."

As well as meeting psychologists to gain insight into the trauma of captivity, Larson wrote three diaries as Ma aged 10, 14 and 17 to help her get into character.

"It was stream of consciousness a lot of the time. I'd get really into it and get stuck in a 10-year-old mind for hours at a time," she says.

"I wanted to create this very full back story for her of what her hopes and dreams were and what her fears were. It might be about body image issues or fights she had with her mum or the boy she had a crush on - the typical growing pains."

After a month writing them and making collages, Larson handed them over to the film's production designer and then didn't see them again until she arrived on set.

The actual set of Room was almost as claustrophobic as it looks in the film.

As Irish director Lenny Abrahamson explains: "The camera lens was always inside the space on a very elaborately constructed set.

"Half the time I was either in the bath or under the sink or on a ladder looking down from above - and very occasionally outside the room watching on a monitor.

"We managed to get most people out so we could shoot with three or four people in there and give the actors some room."

But Abrahamson's biggest challenge was finding a child actor who could play a five-year-old boy raised in captivity.

The audition process led him to Jacob Tremblay, who was just seven when filming began in 2014. As many critics have noted, he and Larson have a remarkable on-screen chemistry as son and mother.

"We had three weeks to bond - for movie-making that's a pretty long rehearsal period," says Larson. "We really got on, we hung out a bunch. We built the toys that you see in Room and spent a couple of hours every day improvising in Room. I'd come over before he went to bed and we'd play with Lego together."

Larson says Tremblay, now nine, has been enjoying all the attention he's been getting at film awards and festivals.

"Sometimes we have to go to fancy parties and he says that all he can see are legs," she recalls. "I worried about him getting tired so I suggested we could have a code word if he didn't want to talk any more - but he said 'nah, I like talking to people, it just gets hard for me after my bedtime'."

With all the Oscar buzz around her performance, Larson is keeping a level head.

"It's like having conversations about your fantasy wedding but you don't even have a boyfriend," she laughs.

"But the cool thing about it is that it's the highest form of recognition you can get in my profession so for people to talk like that is the biggest compliment. If that happens, it's something I'll deal with, but no use in worrying about it right now."

Room is released in the UK on 15 January