Upon seeing Spirited Away (2001), Christian Bale immediately agreed to play any role in this film. He didn't expect to get the titular role.
Although the film was not released in the United Kingdom until 23 September 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 Japanese version Chieko Baishô plays both Young and Grandma Sophie, while in the English dub it is split between Emily Mortimer as Young Sophie and Jean Simmons as Grandma Sophie.
Howl's moving castle is actually centered around War and this is Miyazaki's main message throughout the film. However, interestingly enough, there is no mention at all of war in the novel the movie is based on.
Mamoru Hosoda was selected to direct the film but he abruptly left the project. Hayao Miyazaki then took up the director's role.
In the novel the movie is based on, the black door is actually a portal to Howl's homeland, which is in Wales in our current time and dimension.
Josh Hutcherson who voices Markl and Jena Malone who voices Lettie also star alongside one another in the Hunger Games series of films.
In the novel, Sophie actually 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 beginning of the film, it should have actually been Martha in disguise. Martha is also the character that Markl falls in love with in the book.
This film is an adaptation of a novel of the same name by English author Diana Wynne Jones.