Dogs that portrayed 'Boy' were two Australian kelpies called Hero and Dodge.
When filming with dogs, only Dylan O'Brien was allowed to interact with Hero and Dodge. If Ariana Greenblatt was filming scenes with the dogs, Dylan would have to take a day off and not be present on the set.
The original title of the film was 'Monster Problems.'
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.
Although set in California, the film was actually made in Australia.