Vivienne originally wanted to create a vehicle for all of her friends who were Broadway actors, but couldn't because they were all union or SAG members. She tried to get through all the legal red tape by hiring a producer... who turned out to be a scam artist who absconded with her life savings. Fortunately, she had enough saved up through her commission work through Patreon to start from scratch.
Crymini was redesigned◊, mostly due to the fact that she looked too much like Jay Jay from Zoophobia.
During their development, especially back before Hazbin Hotel became separate from Zoophobia, Angel Dust and Vaggie were in a romantic relationship with one another. This has since changed with both of them being gay and Vaggie being in a relationship with Charlie.
When it came to the development of Charlie's romantic relationships, Viv has explained the original intention prior any discussion of a pilot or a realized cartoon was for Charlie to gain a female love interest while also having an ex-boyfriend and many male crushes. Vaggie wasn't originally going to be that female love interest, however, over time her chemistry with Charlie through development became so much so that a few of the storyboard artists who were working off the pilot's script were boarding with the assumption that Charlie and Vaggie were a couple. As such, Vaggie became Charlie's girlfriend instead of some other character.
Cherri Bomb was originally an entirely different◊ character◊ back before Hazbin Hotel became a separate thing from Zoophobia. She had a completely different character design, her nickname was spelled Cherry with a y, her real name was Isabella, she was an actual angel, and most of all, she was Angel Dust's (late) daughter. Suffice to say, she was completely retooled into Angel's demon girl buddy for Hazbin.
Vaggie underwent a number of designs before her final look in the pilot as best pointed out in this amino post. One of her earliest designs◊, back even before Hazbin became its own thing separate from Zoophobia, was even more clear that she's supposed to be a moth demon with her apparently having actual moth wings. Over time, there were a number of changes made to her design such as changing her hair color◊ and giving her an eyepatch◊. Eventually, however, she was redesigned with the look she has in the pilot.
As explained by Viv in a livestream, Charlie was a completely different character◊ before Hazbin became its own thing. She had a completely different design and personality, and she wasn't the daughter of the Devil. Basically, Viv liked the name Charlie so she kept it for a totally new character that was created. Of special note, Charlie's original character had a large arm made of molten lava/black mass that could change into things. Although this arm, along with the steampunk goggles the original design wore and general overall red color scheme, made Charlie's original character look like she was inspired by Hellboy due to the similarities, Viv explained that this wasn't the case as she had never read Hellboy when she designed Charlie's original concept.
Notably, this is given a Mythology Gag during "The Show Must Go On", where one of Charlie's arms briefly turns into a much bulkier and directly demonic form to block one of Adam's attacks, complete with the same black and molten pattern and markings.
According to YourMovieSucks.org, he was approached to be the composer for the pilot. While he was up for doing it, he was too busy to be able to dedicate his time to the project. He later elaborated that in hindsight that was probably a good thing since he felt the music choices were fantastic and that his own style would have been very different.
In the Pilot cast Live Panel, Viv states that Alastor was originally created as a villain for Zoophobia, but changed her mind in the middle of the story to make a universe centered around him and other characters (Angel Dust, Charlie, Vaggie, etc). He became a much funnier, theatrical, and trickster character as a result.
Originally, Angel Dust's real name was Martin rather than Anthony, though Viv wasn't very serious about it as she didn't consider his name important.
Charlie and her family's surname in the pilot was "Magne", but it was changed to "Morningstar" for the series proper.
Mimzy was originally going to be a main character, something lampshaded in her debut episode where she assumes everyone already knows who she is.
Vaggie was originally envisioned as a legitimate Sinner, specifically a Hooker with a Heart of Gold from El Salvador who died in the 1980's, later changed to 2014, with "Vaggie" only being a nickname and her real name being "Vagatha". Contrast that with her being revealed to be a Fallen Angel in the main series and "Vaggie" being her birth name. Although, it’s still not clear if she is a Heavenborn angel or if she’s a human soul turned into one after she passed away.
Daria Cohen's first draft storyboards for "Alastor's Reprise" show some differences from the final version:
Vaggie's reaction after Alastor dresses her up and slaps her butt is less angry.
Alastor kicks a bucket of paint instead of a skull.
When Alastor grabs Charlie to dance with her at the end, her expressions are less enthusiastic and she takes longer to get into it.
The song ends with Alastor getting knocked down by the door when it explodes.
Originally, Sir Pentious was planned to just be the Starter Villain for the series in the pilot and was implied to have been planned originally to be killed by Alastor. During production however, the writing team grew on him which led him not only to survive, but also to become the Hazbin Hotel's second patron for the entire first season.
The dinosaur-like Overlord Zeezi's name was originally Missi Zilla according to concept art from 2021, presumably having been changed to avoid antagonizing Toho's notoriously protective legal team.
Vivienne Medrano originally wanted the name of each episode in season one to be named after various Broadway shows. However, this ended up being scrapped in the final version.
◊, mostly due to the fact that she looked too much like Jay Jay from Zoophobia.
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