Joan Allen, William H Macy and Jacob Tremblay have been added to the cast of Room, Element Pictures and No Trace Camping’s adaptation of Emma Donoghue’s 2010 bestseller that Lenny Abrahamson is directing. Brie Larson stars in the pic, for which A24 acquired U.S. right at Cannes this year. Shooting is underway at Pinewood Studios and on location in Toronto.

Larson stars as Ma, a woman kidnapped as a teenager who has been held captive for years in a tiny room where she lives with her 5-year-old son Jack. To the boy, it’s the only home he’s ever known. But as Jack’s curiosity is building alongside Ma’s own desperation, she knows that room cannot contain either indefinitely. Donoghue adapted the script from her book, itself partially inspired by the case of Austrian woman Elisabeth Fritzl, who escaped from a basement in 2008 after a 24-year captivity during which she gave birth to seven children.

The pic is part of Element’s first-look deal with Abrahamson after they collaborated on the Sundance premiere Frank starring Michael Fassbender. Before that they made Adam & Paul (with Speers Film), Garage and What Richard Did, along with the TV series Prosperity.

Element’s Ed Guiney is producing Room with David Gross, and Element’s Andrew Lowe and No Trace Camping’s Jeff Arkuss and Jesse Shapira are exec producing with Donoghue and Rose Garnett and Tessa Ross of Film4, which is also aboard with Telefilm Canada and the Irish Film Board.

UTA handled the domestic sale. FilmNation Entertainment is handling international.