Initially, it's hard to believe O'Brien, who also starred in the Maze Runner series, is a nerd someone actually describes as "small." But eventually he gives off a scrappy, heroic Hiccup vibe from How to Train Your Dragon, including realizing that not all monsters are what they seem.
Speaking of influences, the script initially shopped around as "Zombieland with a John Hughes-esque love story at its center," leads Joel to Michael Rooker's expert hunter Clyde, who both looks like and occasionally produces the fed up side-eyes of Woody Harrelson, aka zombie torturer Tallahassee in Zombieland. I immensely enjoyed it!