Terry Crews (Brock Stettman from "The Thanksgiving Special") used to be an American football player.
Once again, Katey Sagal plays an amazing embarrassing mom (cf. 8 Simple Rules for Dating My Teenage Daughter, Recess, and Married... with Children, though Mordecai's mom is more like Spinelli's mom than Cate Hennessynote who isn't that embarrassing a mom or Peg Bundynote who is embarrassing, but doesn't really act like a mom).
In "Terror Tales of the Park IV", Grey DeLisle voices another Japanese girl named "Kimiko".
In "Butt Dial", Rich Fulcher plays an answer phone that's also the leader of the Phone Guardians — exactly the kind of bizarre non-sequitur role he played as a supporting actor on The Mighty Boosh, JG Quintel's inspiration for Regular Show.
Bad Export for You and Banned in China: A lot of episodes have either been edited to pieces or banned outright in Cartoon Network stations airing in the United Kingdom, Australia, the Philippines, and Latin America.
In Poland, Romania, and Hungary, the episode "The Unicorns Have Got to Go" has been banned for reasons unknown (probably because of gross-out humor).
The creator, J.G. Quintel, voices Mordecai and High-Five Ghost after Jeff Bennett was replaced.
Minty Lewis, the voice of Eileen, started out as a writer/storyboard artist on the show. When she wrote the line about the baby sea turtle eggs from "Do Me a Solid" and pitched it, they knew she was perfect for Eileen.
Edited for Syndication: This occasionally happens (and not just in the countries mentioned in Bad Export for You and Banned in China above; America did do some slight editing). In "Prank Callers", when the Master Prank Caller was taking a bath, the shot of the bathroom floor had a condom wrapper on it. In all reruns, it's cropped out/digitally removed
In "Fortune Cookie", the word "chink" (an offensive term for someone who's Chinese) was digitally removed from a dim sum cart that passed by.
In all later airings of "The Power", "How in the H are we gonna fix this S?" is changed to "How in the heck are we gonna fix this stuff?"note though the closed captioning on TV and Netflix has the original line, and the DVD and Blu-Ray version has the actual line intact., and "pissed" is changed to "ticked." The same goes for "you pissed me off" in "Meat Your Maker", but, oddly, "Benson's gonna be pissed" is left intact in "Free Cake." Both instances are odd, considering that Mordecai's "Holy crap, it's real!" and "You're screaming like a fricking maniac" haven't been edited in American reruns of the episode "'Ello, Guv'nur."
In the UK, the infamous line "How in the H are we gonna fix this S?" was changed to "How in the heck are we gonna fix this hole?" (except for Netflix UK, where the line is "Now, how in the H are we going to fix this hole?" which still gets crap past the radar by keeping the T-Word Euphemism for "hell") well before Cartoon Network America decided to edit it.
Keep Circulating the Tapes: Wanna see the uncut version of The Power (you know, the one where Rigby actually says, "Now, how in the H are we gonna fix this S")? Only place you'll get to is on the Slack Pack DVD, as for the uncut versions of Free Cake and Meat Your Maker unless you copied it the first time you're out of luck unless you find it through illegal means.
Name's the Same: Thomas is the name of both the park's billy-goat intern and Death's son. Rigby even says so on the episode where Mordecai and C.J. have to babysit Death's son Thomas and Mordecai calls Rigby to babysit him but he thinks he means Thomas the goat intern.
The Other Darrin: Jeff Bennett voiced High-Five Ghost in his first two appearances instead of Quintel. The only explanation for the actor change (since both Quintel and Bennett are still alive and still doing voice work for the show) is that someone thought Jeff Bennett wasn't right for the role and decided that Quintel would be better (or Bennett missed a rehearsal, Quintel took over, and a mutual agreement was made that Quintel take over for Bennett).
On the Thanksgiving episode, Jennifer Hale and Alastair Duncan voiced Mordecai's mom and dad. On the first episode that focused on them ("Maxin' and Relaxin'"), Hale and Duncan were replaced by Katey Sagal and Ed Begley, Jr.
Talking to Himself: J.G. Quintel voices both Mordecai and High-Five Ghost (after the episodes in which Jeff Bennett voiced him), and Sam Marin voices Benson, Pops, and Muscle Man. Both they and Mark Hamill (Skips) voice several background characters as well.
In "Skips Strikes", Skips was originally suppose to have a different secret. Which was him being friends with the Magic Elements.
The last three minutes of the Halloween special "Terror Tales in the Park" had to be redone because CN deemed it too gory and violent for kids. The end result? Rigby getting egged by a giant egg (which actually is still bad, since a later episode revealed that Rigby is deathly allergic to eggs).
In "Trucker Hall of Fame", the flashback of Muscle Dad telling Muscle Man to take a raincoat with him on his prom was edited because the censors saw Muscle Man's line "Muscle Dad, I don't need no protection from the rain!" as a blatant condom joke (or an endorsement for unprotected sex. Your call).
Also Mordecai's mother was the mom who was critical to Mordecai. She was a Role Model of a mother to everyone else, but not in Mordecai's eyes. Also Rigby's mom is sweet, loving and protective of her son. Those are really interesting ideas, but for some reason, J.G. Quintel and the others don't want to make episodes about Mordecai and Rigby's parents, even through Benton Connor keeps recommending it to them. Although there's a Thanksgiving special that finally introduces Mordecai and Rigby's parents, they only make a brief appearances and we don't see much of their personality, and "Maxin' and Relaxin'" shows Mordecai's parents again (his mom is embarrassing, but well-meaning and his father is the sane one and a bit of a doormat).
Much like the first Halloween special, the second one also had to have its ending changed for being too scary and violent, even for Cartoon Network, though the ending that did air (where the tow truck crashes into a tree and everyone comes out as a ghost) didn't seem like much of an improvement in terms of eschewing death references. Here's some storyboard art showing the ending that did get rejected.
The "lady" the unicorns brought for Mordecai in "The Unicorns Have Got to Go" was originally supposed to be an obviously cross-dressed man in a skimpy bra, panties, fishnet, and heels outfit, but the Cartoon Network censors asked them to replace the man with something less risque (and they did: it was a blonde biker with long hair who kind of looks like Robert Plant from Led Zeppelin)note though the animators could have also made the skimpy bra and panties into a dress, unless the censors were really against having a transvestite on the show.
Quintel originally intended for the set-up of the show to be about Mordecai and Rigby working at a zoo full of humans and was very set on it. The higher-ups decided against it, and Quintel thinks it's actually better that they did.
According to Calvin Wong, they wanted Terry "Hulk" Hogan to voice Muscle Dad.
In "Skips' Story" originally the Guardians of Eternal Youth were going to laugh at Walks' name choice.
In "I Like You Hi", Toby Jones and Owen Dennis intended Yuji to be parody of reality TV personalities, specifically the faux celebrity complex, but the only reality show they both watched was Sweet Genius so Yuji was based off of nothing.
There were some jokes in the episode that had some parts removed that changed the context of the joke.
Yuji's tattoo was originally going to be on his forehead.
When they were casting voices for the main characters, Mark Hamill originally auditioned for Pops but Sam Marin already got the role so J.G. Quintel had him audition for Skips which he got the role.
Write Who You Know: Mordecai is pretty heavily based on series creator J.G. Quintel, who even voices the character with his regular voice. Quintel even stated that Mordecai is himself as a bluejay (which is why Mordecai doesn't act like a bird).