The film took 37 days to shoot at around 58 locations in Seoul, including the back streets of Yongsan, Jongno, Itaewon and a few overpasses and crossroads.
Director Yoon-ki Lee had been in a bookstore in Korea, scanning the display of bestsellers, when a lonely little stack of the Japanese novella One Fine Day by Asuko Taira caught his eye, "like one quiet, lonely person drawn to another." He found the story different from the currently popular trendy Japanese fiction, "There's something old-fashioned about it, something very unique and intimate, it felt like an old, forgotten fable," and thought he'd like to make a film that would touch people's hearts the way the book had touched his. The title was a problem, since many films had the same title. Lee thought of the term "enemy" because it can be used in Korean to refer to lovers, friends, even children.
In adapting the story for the screen, Lee expanded on the novella, adding to the number of people the ex-lovers encounter, and changed some details (including the ending scene).
Music director Jeong-beom Kim composed a jazz film score with influences from 1930-40s American music as well as Latin jazz. The music greatly enhances the atmosphere in the film.