Invidebit voices Monster Kid and the SOUL of Kindness.
Shyner voices both Frisk and Temmie.
Upgraded Moon plays Napstablook and Rouxls.
Xander voices both Flowey and Berdly.
Tom plays Gaster and is one of Spamton's two voices.
Steel voices Sans and Swatch.
In addition to playing Mettaton, Alex originally voiced most of the characters in the Chapter 1 songs for Deltarune the (not) Musical. Later on, Rouxls and Jevil were recast to avoid this.
"Another Medium", which was replaced by "CORE". Alex apparently started writing lyrics for Another Medium but he came up with ones for CORE's tune and liked it better. It was eventually remade as "Evacuate" in the Genocide package.
Before the Musical was rewritten to include a Reset (see the What Could Have Been entry), a song called "You Idiot", which mostly followed the tune of "Your Best Nightmare", would have played between "Fallen Down Reprise 2" and "Don't Give Up".
An inversion, but "Star" from the Genocide Package was cut from the base game, not the musical itself.
Died During Production: Artist Moony passed away of unknown reasons in 2017, not having the opportunity to see her work finished. The Evacuate video was dedicated to her.
Follow the Leader: Several musicals of Undertale followed in the wake of this one, including Undertale Genocide The Musical (which started posting songs before UTM finished), Undertale: The Narrator's Musical (which was in casting process before UTM finished but started releasing songs after), and Undertale the Unauthorized Musical (which is inspired by Portal 2: The (Unauthorized) Musical and thus has parodies of Disney and Broadway songs and also started in January 2018).
The Other Darrin: In Deltarune The (not) Musical, Rouxls Kaard is voiced by Alex Beckham in his first song, and by Upgraded Moon Productions in "It's Pronounced Rules".
The Red Stapler: "Star", an unused track that the Undertale fandom didn't care much for at the time, became much more well-known when it was used as a Villainous Lament for Chara in the Genocide Package. It's helped propel the fandom to seeing Star as one of Chara's themes ever since—the fact that there's a star in real life by that name definitely helps.
Trolling Creator: Man on the Internet is notorious for this, releasing three shitpost versions of Megalovania (and one of Megalo Strike Back) when people wouldn't stop asking.
Man On The Internet decided to stream the full Genocide Package in the lead-up to the last song's release. Then, when time came to play Megalovania, they instead played a series of parody mash-ups and other memey songs. And then, in the subsequent showing of "Undertale the Musical Except It Keeps Getting Faster," the real Megalovania plays (at normal speed) when it hits the Judgment Hall scene.
He responds to the unending requests for "Deltarune The Musical"note Keep in mind that Deltarune, as of the release of the numbers game video, is currently unfinished. with this numbers game; basically, whenever a request for "Deltarune the Musical" was posted as a comment on an unrelated video or as a reply to one of his own comments elsewhere, he pushed back the premiere of THE WORLD REVOLVING one day.note As long as the comment isn't obviously joking or trolling.
It might also apply to the release of "Star". Many fans were expecting him to go right to "Megalovania" after "Floweytale", but he instead released "Star". Then on the day he did release "Megalovania" he first posted the parody song version he used during the Genocide Package livestream before finally posting it proper.
Initially, the Musical would not have featured a Reset (and, by extension, would have lacked the song "An Ending"). Instead, the True Pacifist Route events would have occurred after the Mettaton fight, including the True Lab. Then, the battle with Asgore would occur, followed by all of the main characters coming to stop it. A Cut Song called "You Idiot", an adapted "Your Best Nightmare", would have bridged this and "Don't Give Up".
"Snowdin Carol" and "Bonetrousle" originally had very different lyrics. After numerous accusations of theft despite credits being in the video, they were changed for the full musical.
Why the Fandom Can't Have Nice Things: MOTI's comment on his Re: Undertale Genocide Songs says that he was originally going to release the scratch tracks of the Genocide songs for free to the public, but everyone begging for Genocide songs on every video in the Pacifist Musical "...were just needy and rude and guaranteed that they'd have to pay for it...", referring to his decision to make them Patreon rewards instead. At least the minimum Patreon donation was only $1, and those who couldn't donate just had to wait a bit longer.
Working Title: "Hopes and Dreams" was going to be called "Eternity" (hence the prevalence of the word in the song), but Alex decided to stick with "Hopes and Dreams" for simplicity's sake.
Miscellaneous:
In the first Q&A video, when Xander was asked "Why does Flowey sound like SpongeBob?" his response was, to quote Alex, "incoherent screams of rage for 30 seconds." This was brought back in the anniversary stream where Xander admitted he never really minded the Spongebob comparisons, and even at one point introduced himself as "I voice Spongebob—I mean Flowey".
; basically, whenever a request for "Deltarune the Musical" was posted as a comment on an unrelated video or as a reply to one of his own comments elsewhere, he pushed back the premiere of THE WORLD REVOLVING one day.note 