In the Oscar-nominated Room, actors Brie Larson and Jacob Tremblay spend the first third of the film confined in a tiny, claustrophobic space.

According to Reid Dunlop, it's no illusion: the opening chapters in the tale of a mother and her young son held captive by a sexual predator really were filmed in a 150-square-foot set erected on a production studio in Toronto.

"It was a very small box that was built with removable panels so a camera could go anywhere," said Dunlop, the film's first assistant director, on CBC Ottawa's All In A Day Thursday.

"They wanted to make it as real as possible, address [the characters' confinement] as realistically as possible, and give the actors a real sense of the claustrophobia."

Inspired by the novel by London, Ont.,-based Irish-Canadian author Emma Donoghue, Room was nominated Thursday for Oscars for best picture, best director and best adapted screenplay. 

Larson was also nominated in the best actress category, as the young woman who endures repeated sexual assaults by her and her son's captor before escaping midway through the film.

From a technical standpoint, making the film should have been simple, said Dunlop, whose credits include Canadian television series such as Slings and ArrowsMichael: Tuesdays and Thursdays, and Kids in the Hall: Death Comes to Town.

But the enclosed filming space — combined with the harrowing subject matter — meant the process was anything but easy, said the Chelsea, Que., assistant director.

"It's two people in a room, how hard can that be? But it became very psychologically difficult for a lot of the crew, for a number of reasons," said Dunlop. 

"We shot 15, maybe 23 days in that box. So it was pretty intense."

reid dunlop room oscar nomination director

Reid Dunlop of Chelsea, Que., near Ottawa, was the first assistant director on Room, the Irish-Canadian co-production that's up for four Oscars. (Caitlin Crockard/CBC Ottawa)

Dunlop told All In A Day host Alan Neal he knew Room was going to do well after he saw the finished version — and that belief was reinforced after the film took home the coveted people's choice award at the Toronto International Film Festival.

He praised director Lenny Abrahamson's ability to take Donoghue's novel — told from five-year-old Jack's perspective — and bring it to the screen, while also lamenting that Tremblay wasn't nominated for best actor.

Dunlop also said he was still wrapping his head around the fact that a film he'd worked on was up for multiple Oscars.

"It's a weird feeling, for sure," he said. "It's a first time, and it's sort of trippy."