This marked the first use by Studio Ghibli of digital composition, meaning that elements of a scene were composed using a computer. In this case, it's the flying scene with Baron within Shizuku's story. The scene contains many independent elements, including the minor "planets" and Shizuku's characters. Although all these elements were animated by traditional means, they were combined using computer technology.
The backgrounds, clouds, and flying islands (they are called "Laputa") in the flying sequence were painted by an artist named Naohisa Inoue. He is a former high school art teacher who continued painting pictures of "Iblard" on his own. When he had his exhibition, he sent an invitation to Hayao Miyazaki since Inoue was a huge fan of Miyazaki anime. Of course, he didn't expect anything, but Miyazaki showed up, and he bought one painting (which is now on the wall of the cafeteria at Ghibli). The painting is called "Upward Draft" and looks exactly like the fantasy scene in this film. Then, Miyazaki asked him, "Can you work on our next film?". Inoue also worked in this film as a voice actor. He did one of Mr. Nishi's friends, who joined the "Country Road" jam session (he's the tall one).
The first and only movie directed by Yoshifumi Kondô, who sadly passed away in 1998 of a ruptured aneurysm at age 47.
John Denver: Take Me Home, Country Roads (1971) at the movie's beginning was sung by Dame Olivia Newton-John. Shizuku's versions throughout the film and at the end were sung by Yoko Honna, the voice actress for Shizuku.
When Shizuku is looking for a new book to read in the library, you can see on the bookshelf a book titled "Totoro," this is a reference to one of Hayao Miyazaki's earlier films My Neighbor Totoro (1988).