The story takes place in an alternative 1950's Europe where WWI and WWII never happened. 宫崎骏 has been quoted saying that the fictional city of Kokiro has one side on the shores of the Mediterranean, and the other on the Baltic Sea.
During the production phase, 宫崎骏 and his artists traveled to Sweden to research for the film. The photographs they took of Stockholm and Visby, formed the basis of the fictional city of Koriko. The city also contains elements of Lisbon, Paris, San Francisco, and Milan.
When Kiki first arrives in the city, she is almost hit by a bus bearing the name "Studio Ghibli". This is the name of 宫崎骏's animation studio. "Ghibli" is seen on the side of two other buses; once as she is being questioned by the officer.
The dialogue during a scene in which Kiki is given coffee, changes the beverage to hot chocolate in the Disney release. This was done because Disney thought it was inappropriate for children to drink coffee. The video subtitles also mention hot chocolate, although the Japanese "kohii" ("coffee") is clearly audible on the soundtrack.
Prior to Disney's US release on video in 1998, there was an earlier English dub produced in the early 1990s by Streamline Pictures for in-flight screenings on Japan Airlines flights. This dub also had Jiji voiced by a male voice actor. It was considered a very good dubbing job that was more faithful to the original Japanese version than Disney's dub. It also had brief theatrical screenings at film festivals around the United States before Disney bought the rights. It has not been released on any video format, except for the Japanese LD box-set of the Ghibli films as an alternate audio track.
The four-engined biplane (more precisely, sesquiplane) that Kiki sees during the opening credits is a real aircraft, the Handley-Page HP42. Eight of these planes were commissioned during the 1930s; later they were converted to military use, and all were destroyed by 1941. But since this movie - according to director 宫崎骏 - takes place in a world where World War II never happened, it's plausible that the HP42 would still be in civilian service.
The street name signs in the city have been copied from those used in Stockholm, down to using the names (or parts of names) of existing Stockholm streets and squares in at least two scenes, including "Klara Norra Ky" taken from Klara Norra Kyrkogata (Northern Klara Church Street).
Disney's English subtitle translation for the Japanese track on the US DVD release are actually dub-titles. Interestingly, they are not from Disney's 1998 English version of the film but are from an earlier non-Disney English version from the early 1990's. Tokuma, the Japanese company that Disney negotiated the rights to the Ghibli films with, provided this dub translation to Disney for a subtitled release not aware that it was not an accurate translation. However, the dialogue from this earlier dub is much more faithful to the original Japanese version than Disney's dub is.
At first, Miyazaki was only a producer for the film. The first script was written by 一色伸幸, and 片渊须直 was scheduled to make his debut as a director. But Isshiki soon left the project, as Miyazaki wasn't satisfied with his script. Miyazaki then re-wrote the script, and eventually directed the film, with Katabuchi as an assistant director.
Towards the end of the film, Jiji doesn't speak (in human words, at least) to Kiki anymore, because they grew up, not because she lost a power. In the original book, it is explained that a witch girl and a black cat are raised together from infancy, and that's why they can "talk" to each other (notice that Kiki's mum never talks with Jiji directly). They are able to talk to each other because of their close relationship, rather than magic itself (a cat being able to live so long could be magic, though). Miyazaki made Jiji not be able to talk to Kiki even after she regained her power to show that Kiki has grown, and doesn't need her "other self" anymore.
Originally panned by fans of the original Japanese version, Disney later re-released a second version of their English dub. Unlike the original 1998 dubbing, this 2010 re-vision has gone back and removed several ad-libbed lines, American sound effects and added soundtracks and several comedic lines added by Phil Hartman. This revised dub is considered more faithful than the original 1998 English dubbing, but the sound mix is noticeably more "filtered", with moments like Kiki's screams sounding like they're being passed through a fan.