Grave of the Fireflies author Akiyuki Nosaka said that many offers had been made to create a film version of Grave of the Fireflies.[6] Nosaka argued that "[i]t was impossible to create the barren, scorched earth that's to be the backdrop of the story."[6] He also argued that contemporary children would not be able to convincingly play the characters. Nosaka expressed surprise when an animated version was offered.[6] After seeing the storyboards, Nosaka concluded that it was not possible for such a story to have been made in any method other than animation and expressed surprise in how accurately the rice paddies and townscape were depicted.[6]
Isao Takahata said that he was compelled to film the short story after seeing how the main character, Seita, "was a unique wartime ninth grader."[7] Takahata explained that any wartime story, whether animated or not animated, "tends to be moving and tear-jerking," and that young people develop an "inferiority complex" where they perceive people in wartime eras as being more noble and more able than they are, and therefore the audience believes that the story has nothing to do with them. Takahata argued that he wanted to dispel this mindset.[6] When Nosaka asked if the film characters were "having fun," Takahata answered that he clearly depicted Seita and Setsuko had "substantial" days and that they were "enjoying their days."[8] Takahata said that Setsuko was even more difficult to animate than Seita, and that he had never before depicted a girl younger than five.[6] Takahata said that "[i]n that respect, when you make the book into a movie, Setsuko becomes a tangible person," and said that four-year-olds often become more assertive, self-centered, and try to get their own ways during their ages, and he explained that while one could "have a scene where Seita can't stand that anymore," "that's difficult to incorporate into a story."[9] Takahata explained that the film is from Seita's point of view, "and even objective passages are filtered through his feelings"[8]
Takahata said that he had considered using non-traditional animation methods, but because "the schedule was planned and the movie's release date set, and the staff assembled, it was apparent there was no room for such a trial-and-error approach."[8] Takahata said that he had difficulty animating the scenery since, in Japanese animation, one is "not allowed" to depict Japan in a realistic manner.[6] Animators often traveled to foreign countries to do research on how to depict them, but such research had not been done before for a Japanese setting.[6]
Most of the illustration outlines in the film are in brown, instead of the customary black. Whenever black was used, it was only used when it was absolutely necessary. Color coordinator Michiyo Yasuda said this was done to give the film a softer feel. Yasuda said that until that point it had never been used in an anime before, "and it was done on a challenge."[6] Yasuda explained that brown is more difficult to use than black because it does not contrast as well as black.[6]