Brie Larson on ‘Room’ role, bonding with clever young co-star Jacob Tremblay

TORONTO – The first time Brie Larson realized her young “Room” co-star Jacob Tremblay was a mini-master of his craft was during rehearsals in Toronto.

Before filming began for the Canadian-Irish drama, which is based on the bestselling novel by London, Ont.-based author Emma Donoghue, director Lenny Abrahamson held improv sessions in the shed that’s central to the story.

Tremblay and Larson, who are racking up raves from critics, play mother and son Ma and Jack, who escape captivity in the shed they call Room only to find new challenges on the outside.

Larson said the then-seven-year-old Tremblay had already fully embraced his five-year-old character during rehearsals.

“I was asking him his times tables, like ‘What’s … five times four?'” Larson recalled. “He counted it out on his fingers and then said the wrong answer and I was like, ‘No, it’s 20.’

“We kept going with improv and they yelled ‘cut’ and afterwards he turns to me and he was like: ‘I knew it was 20.’

“That was the moment where I was like, ‘Wow, OK, this is a child who knows exactly what he’s doing.’ He’s really smart.”

Vancouver-born Tremblay is matter-of-fact about the way he approached his character, who was born in the shed and thinks it’s the entire world until they escape.

“I pretended that I was Jack and I thought of what it would be like to be trapped in Room,” the charming nine-year-old said at the Toronto International Film Festival, where “Room” won the People’s Choice Award.

“I thought, ‘Nothing is out of the room.'”

Larson said Donoghue’s novel was the first book to make her cry since she’d read “Where the Red Fern Grows” in fourth grade.

It brought her back to her childhood when she lived with her sister and mom in a tiny apartment in Los Angeles.

“It was a little bit nicer than Room, but we had I think a couple of pairs of jeans and a couple of shirts and a headband and Converse,” said the Sacramento, Calif., native, who recently co-starred in “Trainwreck.”

“But we look back on it as being the time of our lives. We lived off of Top Ramen and our imagination, basically, and my mom did such an incredible job of creating a game out of everything.”

To depict Ma’s malnourishment, Larson went on a restrictive diet and worked out with a trainer.

“So there was the physical nature of that, the lack of vitamin D, because I couldn’t go in the sun, and then on top of it working a 12-hour day,” she said.

“It took a lot to recoup at the end of the day. I needed a lot of sleep.”

To bond with Tremblay, she spent time with him playing games and building with Lego, his favourite pastime.

“He would just ask me a lot of questions,” said Larson. “He was very curious about what my favourite animal was and what my favourite colour was and what was my favourite ‘Star Wars’ movie.”

Tremblay was too young to read the entire script, so he was told the story scene by scene.

“It was an adult one and I don’t really understand adult movies,” he said. “But my mom and my parents told me about the movie.”

Of shooting in the room, the former “Smurfs 2” star marvelled: “It was funny because we had to pretend there was just me and Ma in that very small room, but it’s a small room and there was a bunch of, like, eight cameramen standing there filming us.

“We were all squished together, but sometimes we had to take out walls of the room to fit in the camera, because cameras are big machines.”

“Room” opens in Toronto on Friday, in Vancouver on Oct. 30 and across Canada on Nov. 6.