Brie Larson isolated herself in her home for a month, without access to a phone or the internet, while following a strict diet. She undertook this experience to understand better what Ma and Jack were going through. Larson has explained that, as an introvert who enjoys staying at home, she initially thought her month of isolation would feel like a vacation. However, during the last week, she became very depressed and found herself crying all day.
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 Guerre stellari (1977).
The film was largely shot in sequence to make it easier for Jacob Tremblay to perform as his character evolves.
Despite being an experienced actor, Jacob Tremblay found it difficult to raise his voice at Brie Larson during the scene where he expresses anger over his birthday cake not having candles. To help him, the director, Lenny Abrahamson, had the entire cast and crew jump 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.