Production

Although it has been almost 20 years since the manga's serialization, producer Yūsuke Terada thought that the "flat perspective on human relationships and love" of Shimura's work would be unique to today's age and planned the anime adaptation. In selecting the episodes, since the maximum length of the movie was decided in advance, the concept was set to "a story that reminds us of someone who is no longer around us, but who was once close to us." The four episodes that fit this concept were "Ecchan and Aya", "Mr. Sawa and Yagasaki", "Shin and Sayoko", and "Mika and Shin".^[8] In selecting the episodes, Shimura, the author of the original story, was consulted, but she had no objections because she liked the chosen episodes.^[9]

The main cast was chosen through a large audition, but Takahiro Sakurai and Seiichirō Yamashita were recommended by the director, Satō, who also served as sound director for the film. He demanded restrained acting and instructed the actors not to sigh.^[8] Mikako Komatsu, who played the role of Aya, said, "I was conscious of speaking clearly, but also of acting with realistic tension."^[10] Shimura also witnessed some of the recording process.^[11]

The theme song and the musical accompaniment was provided by the rock band CreepHyp. Initially, CreepHyp was only offered to compose background music for the film, but after reading the manga, Sekaikan Ozaki asked Terada if he could write a theme song as well. Shimura also knew CreepHyp from before and liked some of their songs.^[8] In addition, the theme song Monomane written for this film is a sequel to CreepHyp's previous song Boys END Girls.^[12] In creating the musical accompaniment, Ozaki did not decide on a scene and then write a song, but rather "just picked up a guitar and wrote songs that sounded like Happy-Go-Lucky Days, and then adjusted the scale of the finished songs to fit the necessary scenes." Terada said that this was a completely different way of creating the musical accompaniment to If My Favorite Pop Idol Made It to the Budokan, I Would Die, for which he had been the music producer prior to Happy-Go-Lucky Days.^[8]