Originally, this was supposed to be Leonard Nimoy's final acting role before retiring. However, he liked playing Master Xehanort and being part of the series so much he wanted to continue voice acting, of which he does not consider "acting" or "work". Unfortunately, he only got to reprise his role once for Kingdom Hearts 3D: Dream Drop Distance (2012) before his death in 2015.
A World based on The Jungle Book (1967), was to appear in this game, but was taken out for timing constraints and similarities to the jungle located in the Neverland (Peter Pan (1953)) World. The World was also supposed to appear in the original Kingdom Hearts (2002), but was taken out there due to similarities to the Deep Jungle (Tarzan (1999)) World. The World can still be accessed in this game's code (in the Japanese Version anyway), with areas such as King Louie's throne room, a mountainside on the way to the Man Village, and a river with giant lily flowers to cross to play in.
For the English version, Tetsuya Nomura specifically chose Leonard Nimoy and Mark Hamill to voice Master Xehanort and Master Eraqus respectively. Nomura was a big fan of both [Star Wars: Episode IV - A New Hope (1977)] and [Star Trek (1966)], and wanted to pit the two against each other.
Renders of the demo of Kingdom Hearts 3D: Dream Drop Distance (2012) was left in the game data of the Final Mix version of this game, including Ansem, Seeker of Darkness, Xemnas, Sora, Riku, Kairi, and the Twilight Thorn along with various elements of the demo's logo.
This game would serve as the final Kingdom Hearts game to receive a Final Mix upgrade that was released in 2011 in Japan only just like with Kingdom Hearts and Kingdom Hearts II. Now that the newer gaming systems can use internet technology to perform patch updates on newer games, it is unlikely another Final Mix version will be created.
The North American and Europe releases include a number of new features including Crown Stickers, an extra boss, and additional multi-player options, much like the overseas versions of the original "Kingdom Hearts", which had several new features absent from the original Japanese version (but later included in "Kingdom Hearts Final Mix"). When Birth by Sleep received it's own Final Mix version, these features became available in Japan for that version.
The Blobmob Unversed was designed to be a boss enemy, but due to problems with the boss capacity, it was recycled as a normal enemy. Despite this, defeating one relinquishes a lot of experience points.
It is rumored that Olympus Coliseum was added to emphasize the ten years difference between this game and the first game. This time around, Hercules is shown to be a teenager, where in Kingdom Hearts (2002) he is an adult.
Snow White, Cinderella and Aurora make their appearances for the first time in this game since Kingdom Hearts. This is also the first time that are voiced, as they spoke in dialogue bubbles in Kingdom Hearts (2002)
The amount of upgrades released as part of Birth by Sleep's Final Mix version is the least amount of upgrades compared to the previous Final Mix versions, Kingdom Hearts Final Mix and Kingdom Hearts II: Final Mix+ (2007). As a result it lacks a number of common Final Mix features such as new regular enemies, and new synthesis items in the Moogle Shop (as purchasing Commands replaced the Item synthesis for Birth by Sleep).
Donald and Goofy are shown to be at the Mysterious Tower, despite not recognizing it in Kingdom Hearts II (2005). This is due to the floors being constructed differently. Also, according to Nomura, the Mysterious Tower function as a world is unique, which it why it is constantly changing locations in each game.
Originally, past versions of the Final Fantasy characters, namely Leon and Cloud, were going to appear in Radiant Garden, but were cut out of the final release due to the world already having a lot of characters already, which might have made things confusing.
Sora and Riku's appearance in the games' secret ending (in the present, which takes place after Kingdom Hearts II (2005)) about being summoned by King Mickey is a tie-in to about their being summoned to undergo the Mark of Mastery Exam which would take place in Kingdom Hearts 3D: Dream Drop Distance (2012).
While the Heartless do not appear in the main game as the Unversed play that role instead, a few Darkside Heartless appear in the ending during the Final Episode. In addition for Birth by Sleep Final Mix only, the player can use Aqua to fight against regular Heartless in the Final Mix-exclusive Secret Episode, namely Shadow, Darkball, and Neo-Shadow Heartless.
When Kairi is being attacked by the Unversed, she places her hand on Aqua's Keyblade unintentionally. Like with Terra and Riku, this triggered a Keyblade Inheritance Ceremony that gave Kairi the ability to wield a Keyblade, as shown in Kingdom Hearts II (2005)
The music that is playing during Terra's final battle against Xehanort is the same music played when Sora, Donald, and Goofy fought against Lingering Will from Kingdom Hearts II: Final Mix+ (2007). Also the concept of Terra's spirit inhabiting his Keyblade armor (after Xehanort took over his body) is also a reference to that battle, as he is the Lingering Will boss in that game.
The Secret Episode which is exclusive to Birth by Sleep Final Mix is a new adventure involving the Aqua's journey in the Realm of Darkness has a chapter named "A Fragmentary Passage". This is also one of the titles in Kingdom Hearts HD 2.8 Final Chapter Prologue (2017), as the one presented in the latter title is actually a continuation of the story that was started here. This is also the only part to have new cutscenes, as unlike the previous Final Mix titles, the new cutscenes only take place during this hidden post-game event instead of the main game. Also like with the Final Mix-exclusive cutscenes from Kingdom Hearts II: Final Mix+ (2007), this scene is presented in Japanese by Aqua's seiyuu Megumi Toyoguchi, but was later dubbed in English by her English voice actress Willa Holland with the release of Kingdom Hearts HD 2.5 Remix (2014).
The Game's secret unlockable ending by completing Final Episode which is known as Blank Points shows what happens to various characters including a scene of Sora, Riku, and Kairi in the present day. When Birth by Sleep Final Mix came out, it was added as part of the Secret Episode save file and helps serve as a prelude to the Secret Episode mode.
According to the novels, Vanitas resembles a teenage Sora due to the latter filling the void in Ventus' heart after Vanitas was extracted from it. Prior to this connection, Vanitas resembled a Heartless with red eyes.
According to Nomura, the hero that Terra reminds Zack of is Sephiroth. Sephiorth originally appeared in Kingdom Hearts II (2005) as an antagonist, but since Birth by Sleep takes place over a decade in the past, it's possibly he hadn't become a being of darkness yet.