Mickey Mouse is voiced here not by his mainline actor, Bret Iwan, but by Chris Diamantopoulos, who usually only voices him in the Paul Rudish shorts. Diamantopoulos was cast in said shorts because Rudish and his team wanted someone who could emulate Walt Disney's original performances. As such, casting him in this short makes perfect sense because the picture that Mickey springs from is from 1942's Mickey's Birthday Party, where he was voiced by Walt.
One of the characters Snow White holds hands with during her solo line in "When You Wish Upon a Star" is Asha, the heroine of the then-upcoming Disney film Wish. The heroine of the first Disney movie is giving her blessing to the heroine of the latest (at the time) Disney movie! The trio is rounded out by Mulan, whose movie premiered the same year as one of the studio's previousMilestone Celebrations, 1998's 75th Anniversary campaign.
Meta Brilliance: When he's being called to come join the picture, Olaf from Frozen is drawing a picture of the Genie from Aladdin and singing a line from "Friend Like Me"; Josh Gad has stated that the Genie (and especially Robin Williams' performance as him) was a big influence for his performance as Olaf. In a way, Olaf is showing his love to his inspiration.
Also, the book Olaf's using as a booster seat? The Illusion of Life by Frank Thomas and Ollie Johnson of Disney's Nine Old Men, which is a book about character animation, the very thing Olaf's doing! If Frozen Fever indicated anything, he didn't actually read it, though...
When Scuttle is trying to sing along during the big performance of "When You Wish Upon A Star", everyone around him looks uncomfortable, but Dolores flinches like she's in pain. Considering that her gift is super hearing, Scuttle's harsh "vocals" probably were a little painful for her.
Two of the more Unexpected Characters in the short are Wayne and Lanny from the Prep & Landing tv specials. Then you realize that the magic of Disney and the magic of Christmas usually relate to the same thing-bringing joy to people and Wayne and Lanny's whole job is making sure that Santa can arrive to bring Christmas magic and joy. Thus, it actually makes perfect sense for Wayne and Lanny to be present, considering that they hit that perfect overlap between the two types of magic.
The title character of an Ugly Duckling short film from 1939 is standing right next to Lilo & Stitch, who bond over being able to relate to an Ugly Duckling storybook.
As funny as it is for being unexpected, Splat having a crush on Ursula actually has more of a solid basis than one might think when one considers certain details from the former's home movie. Specifically, in Strange World, Splat's species is shown to be in a symbiotic relationship with the distinctly cephalopod-like reapers (with cells like Splat guiding the reapers towards 'impurities' that the reapers subsequently eat). And considering how Ursula is half octopus, she likely reminds Splat of the reapers he and his kind help guide to 'food' back in his homeland, hence why he's so swiftly drawn to her.
One of the Deleted Scenes was Captain Hook getting run over by Tuk-Tuk. While this might seem odd, Tuk-Tuk sounds a fair bit like Tick-Tock, referencing Captain Hook's troubles with the Tick-Tock Croc.
Ariel is portrayed here as being a human instead of a mermaid because otherwise she wouldn't be able to leave the building, as best demonstrated when we first see Flounder in the short. This may also explain why King Triton doesn't appear here.
Why is Goofy chosen to take the picture? Well, he's had experience before such as in Hold That Pose! and A Goofy Movie.
Kaa hypnotizing Clarabelle Cow was undoubtedly funny, but think about this: Kaa generally only hypnotizes people he intends to eat. Had Rapunzel not whacked him with her frying pan, Kaa would have eaten a beloved character who's been part of the Disney family for decades! No wonder Mickey was so panicked when he saw it happening.
Considering the nonchalance Rapunzel treats it with, this is probably something that happens all the time.
Upon closer inspection of the characters, one of the ones from The Lion King (1994) that isn't there is Mufasa. But everyone else is there, including Scar. It's possible that the lion may have made sure his brother couldn't make it to the big group photo by arranging a little "accident", and Simba could walk back into the studio to find his father's lifeless body lying on the floor all over again.
If it helps, perhaps there just wasn't a picture of Mufasa anywhere in the studio building, and was consequently unable to "come to life".
