Dogs that portrayed 'Boy' were two Australian kelpies called Hero and Dodge.
When filming with the dogs, only Dylan O'Brien was allowed to interact with Hero and Dodge. If Ariana Greenblatt was filming scenes with them, O'Brien would take a day off and not be present on set.
The original title of the film was 'Monster Problems.'
Although set in California, the film was actually made in Australia.
Joel wears a t-shirt with palm trees on it which has the same design as the t-shirt Ellie wears in the video game, The Last of Us (2013). That game takes place in a post-apocalyptic world. "Joel" is also the main protagonist's name in the game.