Kathryn Beaumont, who voiced Alice and Wendy in Alice in Wonderland and Peter Pan, respectively, reprised her roles despite being in her mid '60s at the time. She's also one of only a few actors/actresses who worked with Walt Disney directly to be involved with the Kingdom Hearts games as well; John Fiedler, who voiced Piglet, did the role in the games as well until his death.
Jodi Benson, who was 31 at the time, once again voiced the 16-year-old Ariel.
Development Gag: Rikku from Final Fantasy X was going to make an appearance as part of the Final Fantasy gang in Traverse Town, but she was replaced with Yuffie, who uses the costume originally designed for the KH version of Rikku.
Kevin Michael Richardson replaced Samuel E. Wright as the voice of Sebastian, the only time the character has been voiced by someone other than Wright.
Due to the characters being much younger than their canon counterparts, Tidus and Wakka have different voice actors. Tidus was voiced by Sean Fleming instead of James Arnold Taylor, while Wakka was voiced by Dee Bradley Baker instead of John DiMaggio.
There was planned to be one more battle against Ansem-possessed Riku at the End of the World. It was very nearly completed but Dummied Out in the final version.
The original scenario involved Sora saving the Princesses of Heart with the assistance of their respective princes.
Kairi's role in Hollow Bastion was going to be larger.
Bahamut, a recurring Final Fantasy character usually included as a Summon, was originally going to be featured. While it is possible to hack the game to include Bahamut in the Summons menu, choosing him will crash the game.
The original concept for the game had Mickey as the main character, with Donald and Goofy helping out. The development team eventually became divided on whether he or Donald should be the main character, and so they compromised by creating Sora.
Sora's original design was more lion-like and wielded a chainsaw sword. This was changed to a giant key to be less violent.
Many early screenshots for the game showed Donald and Goofy wearing their classic outfits from the original Disney shorts. This was changed to the current attire they wear in the game, presumably to give them more adventurous appearances, and they can only be seen wearing their classic attire in the game's epilogue.
Chernabog was going to be the final boss, being the embodiment of all darkness. This was changed for various reasons. Some concept art shows that the battle would take place at an unknown castle.
Some concept art showed Sora wearing a loincloth in the Deep Jungle and a world icon for a Toy Story world.
A world based on The Lion King was planned, but Square did not have the technology necessary to make Sora's lion model function properly. Instead, Simba only appears as a summon and the world was eventually featured in Kingdom Hearts II.
"Destati" is apparently considered cursed by the production team due to odd incidents—the worst being the entire building losing power—occurring whenever the vocal track was inserted into a piece of background music.
Kurt Zisa, one of the game's bonus bosses, is named after a fan who won a contest. He appears if you try to fly to the Cave of Wonders from Aladdin's house after visiting Hollow Bastion.
For the remastered version of the Final Mix re-releases, Nomura himself stated that the team had to rewrite the whole code of the game from what was available from the original game since the original version's game data was somehow lost, which lead to a ton of research, and stated that "it wasn't easy trying to recreate everything in HD from there".