It took two weeks to assemble Walt Disney's voice tracks to make Mickey exclaim "Red!" and make it sound surprised.
Look for a cameo by Oswald the Lucky Rabbit. He appears near the end peering from behind the torn screen.
When the producers were going through all the archive recordings of Walt Disney, they discovered he never said the word "red" while voice acting as Mickey. This proved to be a problem, as there was going to be a moment when he would comment on the color of his shorts upon entering the 3D world. To solve this, the sound editors "created" the word red by splicing together three different sounds, an "r" sound, an "eh" sound, and a soft "d" sound, each taken from a different line, thus, creating the word "red" as said by Mickey. If you listen very closely when Mickey says the line, you can hear a slight jarring from the lines being spliced together when he says "red".
The producers went through every single Mickey Mouse cartoon produced between 1928 and 1947 to get archive recordings of Walt Disney voice.