Upon seeing A Viagem de Chihiro (2001), Christian Bale immediately agreed to play any role in this film. He didn't expect to get the titular role.
Sophie's city was modelled after Colmar, France.
Although the film was not released in the United Kingdom until September 23rd, 2005, director Hayao Miyazaki personally traveled to England in the summer of 2004 to give a private showing of the film to Diana Wynne Jones.
Hayao Miyazaki and Lauren Bacall, both long-time fans of each other's work, met at a subtitled screening in New York. Reportedly, Bacall jokingly asked Miyazaki if he was married.
In the novel, Sophie has two sisters. The second is named Martha, who is sent to be a witch's apprentice but finds a spell to change her appearance and switch with Lettie so that she can work in the bakery instead. So when Sophie goes to see "Lettie" at the film's beginning, it should have been Martha in disguise. Martha is also the character that Markl falls in love with in the book. In the movie, Martha is mentioned by the shop girls, saying that Howl ate Martha's heart, but is not presented as Sophie's sister.