Acting for Two: Zach Hadel has voiced a few characters that Charlie's interacted with, such as DJ Spit, Mip and Satan. Michael Cusack also voiced both Alan and Pim, who have spoken to each other a few times.
Approval of God: Someone had made an edit of the 'S-S-S-Secret Room' clip from "Who Violently Murdered Simon S. Salty?" that ended with Pim and Charlie passionately kissing, claiming it was a deleted scene from the episode, Zach Hadel has seen the edit and laments that he didn't come up with it first.
Many sources mistakenly say that Mip's design is based on Ralph Bakshi's adaptation of The Lord of the Rings. He was actually based on Rankin/Bass Productions' adaptation of The Hobbit, though given that many mix up both Tolkien adaptations due to them being released around a similar time period, the confusion is understandable.
A viral negative review of the series identifies Michael Cusack as "PsychicPebbles", which is actually Zach Hadel's online handle, and it seems to be under the impression that the two are one person, even though both are explicitly credited on the opening title card. This is such an elementary mistake that it has led Cusack and Hadel to opine that they think the review might be a troll.
Descended Creator: Creators Zach Hadel and Michael Cusack lend their voices to various characters throughout the show. Among the main cast, Hadel voices Charlie and Glep while Cusack voices Pim and Alan.
A positive example in that [adult swim] made the creators slow down a scene of a dancing biliblie to prevent potential seizures.
DJ Spit originally was supposed to have visible pubic hair, but the network objected to this. The objection was not made, however, until the episode had already been fully animated, meaning that someone had to go through each individual frame and edit them. Zach joked that if there'd have been time, he would have found the person tasked with this and credited them as "pube eraser".
Mustard was supposed to give Salty a blowjob (offscreen), with the joke being that she was sucking impossibly fast. The network axed this, and the joke was later reused in the season finale with Charlie's grandma and a lollipop.
In the season finale, Charlie was supposed to take a hit from a vape pen at the beginning. Satan taking a hit from his own vape later in the episode was meant as a Call-Back to this, showing that the two characters are stuck in similar depressive funks. Due to Adult Swim being partnered with an anti-vape company, higher-ups objected to this. Charlie vaping had to be cut entirely, and Satan was only allowed to vape after the smoke he blew out was changed in color to red.
Fake Nationality: Australian Michael Cusack uses an American accent for Allan and James (and can be heard wobbling in the latter instance when James contemplates whether or not to "sawr" off Charlie's head), while American Zach Hadel uses a British accent for Mip and a Hispanic accent for DJ Spit.
While the entire pilot was made in the USA, half of the series production including animation, financing and storyboards are done at Princess Bento in Australia.
Mr. Man, the one who agreed with Grim and Gnarly's plan to kill the Puerto Ricans in "Frowning Friends", is voiced by Lyle Rath, who is of Puerto Rican descent.
At the end of "Charlie Dies and Doesn't Come back", Elderly Glep, who ends the episode by saying that Christianity was right all along, is voiced by the Jewish Joshua Tomar.
No Budget: According to Co Creator, Zach Hadel, the budget for the entire first season (Which consisted of nine eleven minute episodes) was equivalent to that of one Family Guy episode (2 million USD).
Out of Holiday Episode: Not only did "A Silly Halloween Special" and the Christmas themed "Charlie Dies and Doesn't Come Back" not come out around their respective holidays, but they came out the very same night along with all the other episodes. While the originally planned release in mid November would've put the Christmas episode as coming out in December (unless they all debuted in one night like they ultimately did in January) the Halloween episode still would've been some time after the holiday. In addition, "The Smiling Friends Go To Brazil" is set during Mardi Gras, which is usually in February (sometimes early March), but didn't air until August, about as far away from Mardi Gras as you can get.
After fans noticed James Rolfe's name in the credits of "Charlie Dies and Doesn't Come Back", many believed that he had voiced Satan in the episode. It later turned out Satan was voiced by Zach, albeit with his voice pitched down to the point where he was nearly unrecognizable, and Rolfe actually made a brief live-action cameo appearance during Charlie's hallucinations upon entering Hell.
The grease puddle is not voiced by Chris-Chan; it is voiced by Harry Partridge (who's a fan of Sonichu) doing an impression of Chris-Chan.
Studio Hop: Animation duties went from 6 Point Harness and Studio Yotta for the pilot, to Princess Bento for the series proper. However, Yotta would return for "The Smiling Friends Go To Brazil," with Princess Bento receiving production credit.
The reason Mr. Man inexplicably transforms into DJ Spit is because Zach and Michael were rapidly closing in on their deadline and couldn't think of a better way to end the scene.
Chris O'Neill mentions that the discordant, dial tone-like notes in the show's ending theme were made up by him hitting random keys on the music program as the original theme played; they fit well together and was in line with the zany, jarring nature of the show itself, so it was left in.
The "Renaissance Men" bit from "Frowning Friends" was entirely ad-libbed. Notably, Zach says that the conversation was recorded late at night after a long workday, and that you can hear how exhausted both he and Michael were in their voices.
Word of God: Per commentary tracks on the season one Blu-Ray by Zach and Michael, new info on the series has been revealed:
Mr. Boss's kid, who he breastfeeds in the pilot, is named Jason.
Contrary to his name, Mr. Frog is actually a demon and not a frog.
The man giving Charlie and Shrimp form tips is based on an actual experience Cusack and Hadel had at the gym.
Smormu was meant to be the same size as Charlie, but Cusack and Hadel found it funnier that he was mistakingly drawn the same size as Pim, so it was kept in.
Allan's last name is "Red", they discussed what Glep's last name could be but they forgot what it was.
on the season one Blu-Ray by Zach and Michael, new info on the series has been revealed: