On 19 December 2018, it was announced that Lukas Dhont was developing a follow-up feature film to his 2018 Cannes Film Festival award-winning directorial debut Girl. Co-writer Angelo Tijssens and producer Dirk Impens were attached to re-team with Dhont for the then-untitled film, with Dhont stating that "It's going to be different but also in the style of Girl", and that "At the centre of it is a queer character." In writing the film, Dhont was inspired by psychologist Niobe Way's book, Deep Secrets: Boys’ Friendships and the Crisis of Connection, which documents her study of intimacy among teenage boys. Dhont named the film after a "close friendship," a recurring term in the book.
