Hirokazu Koreeda wrote the first draft of the screenplay fifteen years before the film was actually made. At that point it was titled "Wonderful Sunday" and unfolded from Akira's subjective point of view, ending with a fantasy sequence in which the entire family (the children, the mother and the various fathers) are reunited for a Sunday outing.
According to the director Hirokazu Koreeda, though Nobody Knows (2004) was inspired by the true story of the Sugamo child abandonment case, it is not a factual recounting, and only the settings and the ending of the story are based on the true story. Also, the film's version of the story was far less grisly than the actual event upon which the movie is based on.
The director Hirokazu Koreeda held extensive auditions to cast the four children, and the actors were all nonprofessionals. Also, during the casting, a little girl came in with noisy sandals. The director liked it so much that he brought it over to Yuki's character when searching for her mother. He also did not give the children detailed explanations of their roles, because he wanted them to be natural.
Director Hirokazu Koreeda said that during the long filming period, he tried to build a relationship of trust between himself and the children, and also amongst the children themselves. During the children's filming breaks, the children were asked to write in their own journal entries about what they were thinking, ranging from the film to their own everyday concerns.