Brie Larson isolated herself in her home for a month without a phone or internet and followed a strict diet to understand what Ma and Jack were going through. Larson has said that because she considers herself an introvert who prefers to stay at home, she thought that her month of isolation would be a vacation, but towards the last week, she became very depressed and would cry all day.
Jacob Tremblay, though an experienced actor, could not bring himself to yell at Brie Larson in the scene where he is angry about his birthday cake having no candles. Finally, the director Lenny Abrahamson had the entire cast and crew start jumping up and down, yelling and screaming until he could do it himself.
Brie Larson claimed that she avoided washing her face during filming to make it clear on-camera that she was not wearing makeup.
When trying to get them to bond, Jacob Tremblay's mother asked him if he had any questions for Brie Larson. He asked her three questions: her favorite color, animal, and whether or not she liked Star Wars: Episode IV - A New Hope (1977).
The film was largely shot in sequence to make it easier for Jacob Tremblay to perform as his character evolves.