In 2021, a 15-second clip of beta animations for the game dating back to 2006-2007 was discovered on the website of animator Michael Daubert, a former employee of the outsourcing studio The Animation Farm who produced animations for an internal pitch of an early iteration of the game. These tests showcase a completely different art style from the final game and unused gameplay elements, with Mickey shooting ink out of his body to quickly surf around, and jumping and slamming the ground to summon a tidal wave of ink to attack large Blotling enemies. This tidal wave attack also causes Mickey to lose all of his colors, turn white and become exhausted, suggesting that the known early idea of ink/paint/thinner usage changing his physical appearance was going to involve a stamina meter tied to his attacks. Mickey's early design was more stylized compared to his traditional appearance in the final game, featuring a prominently tall forehead, a lanky, stretched body, no visible mouth, and crooked oval ears. This early design would live on through an emblem used in promotional materials and as a recurring icon throughout the final game, with the inky black outline of the early design's head forming an "M".

In 2024, an extended pitch trailer for this version of the game using some of the same animation footage was posted online by Thomas Heimann, another former employee of The Animation Farm. The pitch for this version of the game proposed a 2008 release date and outlined possible gameplay mechanics, an early version of the story, and early character designs for Mickey, the Phantom Blot, and the Blotlings. A model sheet for Oswald the Lucky Rabbit with character designs dating back to his initial usage in the 1920s-30s was also teased during the trailer, but he does not appear in it.

Within the game files are unfinished videos for two unused sketches: the balloon and the fireworks. It’s likely the fireworks sketch became the fireworks mechanic in Epic Mickey 2: The Power of Two.

During development of Epic Mickey, Warren Spector used clips from various Mickey Mouse cartoons from the golden age as reference for Mickey's character animation. By recreating some scenes from some of the cartoons and overlaying it with the cartoon, the team were informed of how Mickey should move throughout the game.

The music that plays when Yen Sid is seen painting the Wasteland is Paul Duka's musical adaption of the poem "The Sorcerer's Apprentice" popularized by Fantasia, the 1940 Disney animated movie where Yen Sid made his first appearance.

The Mickey Junk Mountain area is based off of the Matterhorn attraction from Disneyland. But Instead of snow, its made out of actual Mickey Mouse merchandise produced throughout the years.

In Mickey Junk Mountain, a can with Donald Duck holding a glass of juice is seen among the piles of junk in the game. This is a reference to a brand of orange juice from the Florida's Natural Growers agricultural cooperative that used Donald Duck to market its orange juice, even using Donald as its mascot.

During development, the bunny children came in four different colors: black, white, light red, and light blue. The final game features only light blue bunnies.

In the E3 trailer of Epic Mickey, the Mad Doctor is seen in the black cape uniform like in the cartoon 'The Mad Doctor'. This was changed in the final game to avoid confusing him with the shadow blot.

The official Epic Mickey strategy guide misidentifies Gremlin Trigo as Gremlin Tringo.

Spatters were originally going to be a variety of colors, as was the splat of paint on their heads.

When first meeting Gremlin Calvin in Dark Beauty Castle, he is wearing a yellow suit. When saved, he reappears in Gremlin Village, now sporting a green suit.

It was originally intended for Wonderland to appear in the game. The world can still be seen in the opening and closing cinematics.

Mickey is aided throughout the game by Gus and other creatures based off of "The Gremlins", an unproduced film collaboration between Disney and author Roald Dahl that dates back to World War II.

According to Warren Spector, there were originally three different designs for Mickey, with his appearance changing based on whether the player made heroic or mischievous choices. The designs were based off different eras of Mickey Mouse cartoons. Hero Mickey was based on his old movie posters, Wasterlander was based on 1930s vintage Mickey, and Scrapper was inspired by the early black and white Mickey cartoons (along with the 1995 Mickey Mouse short 'Runaway Brain').

Mickey was originally to have released his own paint from his hands. As he did this, the paint from his body would vanish, revealing his outline. The idea was later scrapped, and Mickey was given a paintbrush instead.

Pluto's bowl and bone can be seen in Mickey's house.

In an early version, Gus was to be caged like all the other Gremlins in the game as opposed to being free by default. This may explain why Gus never introduces himself to Mickey, as the scene where he would have done so was cut and never replaced.

Originally, if Mickey used too much thinner when he was in his 'scrapper' mood, he would've turned into a design called 'Thinner Mickey'. Every evil action that Mickey would have done erased his paint and showed more of his basic line-art. The idea was eventually scrapped due to the Disney company not wishing to tarnish Mickey's 'heroic' attitude.

In the original script, Oswald was going to be the main antagonist, but was replaced with the Blot in order to avoid hurting his reputation by portraying him as an unlikable character.

During the ending, Spoiler:the Gremlins use a recolored model of Gus. Due to this, every Gremlin has Gus' hat and mustache.