Frank's makeup job from the mortuary in "Frank Reynolds' Little Beauties" makes him look like The Penguin in Batman Returns, one of Danny DeVito's more famous film roles.
The scientist in "Flowers for Charlie" was played by Burn Gorman, Charlie Day's co-star in Pacific Rim.
Speaking of, it took a few years, but we finally saw Charlie dress up as a scientist talking about science in the rebranded Fight Milk commercial in "Wolf Cola: A PR Nightmare".
Da Maniac, played by Roddy Piper, almost always wears a black leather jacket similar to the one he often wore as a wrestler.
California Doubling: While some exteriors are shot in Philadelphia, the majority of the show is shot in Southern California. This explains why's it's so suspiciously "always sunny in Philadelphia."
The exterior of Paddy's Pub is actually the Pan Pacific Warehouse in the Arts District of Los Angeles. Although the area is almost completely industrial, it is just down the street from Villains Tavern.
One particularly ironic example is in the end of "Psycho Pete Returns," where the gang puts Pete on a train bound to Los Angeles while obviously standing in Los Angeles Union Station.
The Cast Showoff: Charlie, like his actor, can sing and play piano. He's also great at ice skating, as Day used to play hockey.
It's not just limited to Charlie Day: Kaitlin Olson, Glenn Howerton, and Rob McElhenney all get the chance to show off their pleasant singing voices in a few episodes.
Channel Hop: In an effort to attract interest in the launch of FX's comedy-oriented spinoff channel FXX, Sunny was moved to the new channel along with The League and Legit.
Corpsing: With everyone in the main cast prone to ad-libs, it happens constantly on set (Danny cracks up the least, while Rob does it the most); however, almost none of it gets into the finished product.
The Danza: Charlie Kelly is played by Charlie Day, Mac is played by Rob McElhenny, and recurring character Artemis is played by Artemis Pebdani. Specifically averted by Dennis: Glenn Howerton did not want his real name associated so closely with the character.
Those dick towels they were trying to market? You can buy them, but don't Google them if you're on a shared home computer or on a computer in a public setting (read: library, computer lab at school, etc).
Greenman, which has not only been defictionalized, but is well on its way to cultural phenomenon status.
Recipes for "milk steak", a dish Charlie apparently made up in "The Waitress Is Getting Married", have appeared online.
And now fans have done the same for Rum Ham (not to be confused with an existing recipe for wussy, rum-glazed ham). It tastes pretty awful, but it'll get you good and wasted.
Development Gag: A subtle one in "The Gang Beats Boggs". The show was originally going to be set in Los Angeles and named "It's Always Sunny on Television", in reference to LA's picturesque weather. When Charlie and Mac get to LA in "The Gang Beats Boggs", Mac pointedly states how gloomy the overcast city looks.
Dyeing for Your Art: Rob McElhenney gained 50 pounds in order to prove a point about sitcoms for season 7.
Exiled from Continuity: Averted; Rob McElhenney got the idea for Charlie's "Green Man" persona from an unnamed friend, who donned a green zentai suit while attending a football game with McElhenney. McElhenney would later pay his friend an undisclosed amount of money in exchange for the rights to his "character", so that he could use the concept in later episodes.
"Funny Aneurysm" Moment: Stephen Collins plays the altruistic biological father of Dennis and Dee, who is active in children's charities and adopted a dozen African orphans. In 2014, Collins was recorded confessing to the molestation of multiple children.
Dee's pregnancy was written in because Kaitlin Olson was pregnant. The show did hide her bump before it was revealed she was pregnant. They actually pulled it off, though; watch "Mac and Charlie: White Trash" for some subtle hiding.
Mary Elizabeth Ellis doesn't appear much in Season 7 and wears a loose dress to hide her pregnancy.
Incestuous Casting: A minor case. Lucy DeVito, Danny DeVito's daughter, plays the waitress whom Frank ogles in "Mac Bangs Dennis' Mom."
Dee is the resident Butt-Monkey of the group and is constantly abused by everyone. Kaitlin Olson is married to Rob, the show's creator.
Even more ironic, there's two scenes where Dee and Mac have to pretend to be into each other and act very unconvincingly. One time, they act out a Sitcom "Will They or Won't They?" subplot to help popularize their bar. It ended in a fight where Mac strangles her. Another time, Dee plays the Love Interest of Mac's character in a film which worked out fine until they had to kiss.
A fan theory is that The Waitress's name is Nikki Potnick because she couldn't find her name tag at her high school reunion and Frank had stolen the "Nikki Potnick" tag. During his "Ask Me Anything" for Reddit, Glenn Howerton said in no uncertain terms that "the Waitress is NOT Nikki Potnick."note The name "Nikki Potnick" is also mentioned in the first season episode "Underage Drinking". Since the entire gang knows who that is, and nobody seems to know what the Waitress's real name is, it can be safely assumed that the two aren't the same person.
Another fan theory was that the letters for "Pepe Silvia" were actually letters addressed to "Pennsylvania," a misunderstanding brought about by Charlie's illiteracy. The writers claimed that this was entirely coincidental.
Mean Character, Nice Actor: The main cast kind of have to be this trope, givenwhotheyhavetoplay. Special mention goes to Glenn Howerton, who finds Dennis to be such a repulsive psychopath that he opted out of being The Danza like Charlie and Rob, not wanted to be too associated with someone like Dennis.
Nice Character, Mean Actor: Stephen Collins, who plays Dennis and Dee's saintly biological father, was later charged with child molestation.
No Budget: The show's first season was produced at 1/3 of a typical network sitcom's budget, starred unknown actors and during production, Rob McElhenney still kept his day job as a waiter. Eventually, the budget increased with each successive season.
Future Joss Whedon Production Posse member Fran Kranz has a small role in "Who Pooped the Bed?" as the Econ student who just wants to see some poop.
Jaimie Alexander, who would later star as Sif in the Thor movies and the lead in Blindspot, shows up in Episode 3 as a teenage girl who flirts with Dennis to make her boyfriend jealous.
Schedule Slip: Season 10 did not premiere in fall 2014, instead premiering in January 2015.
Stunt Casting: FX executives told Rob Mc Elhenney that they would cancel the show unless they brought on a famous actor. They eventually settled on DeVito. This is one of the most successful examples of stunt casting in television history, turning a show with middling reviews and poor ratings into an acclaimed ratings hit.
Throw It In!: A lot of the dialogue, especially the Gang's arguments with each other, is improvised or only partially scripted. This explains why Dee has so many one-on-one scenes with Charlie: Kaitlin Olson, her actress, is the best among the group at not Corpsing at Charlie Day's bizarre speeches.
Unintentional Period Piece: Thanks to the large number of references to the then current events of the Turn of the Millennium. The show debuted during that decade, as such many episodes make references and jokes that seem to be ripped from that decade's headlines, such as references to the gas crisis, terrorism, waterboarding, Hurricane Katrina, North Korea, the bar getting a new flatscreen TV and Blu-ray player, just to name a few.
According to Rob's AMA, Ryan Dunn was in talks with the cast to appear in season 7, but he died before anything could be set in stone or filmed.
Miscellaneous fun facts:
When series creator Rob McElhenney was asked a question about his character Mac in a featurette, he replied "I don't know... I just show up, say my fuckin' lines, and go home."
Charlie's play The Nightman Cometh was for a while actually touring various cities around Philadelphia with all of the original cast members.
According to Charlie Day, during a recent interview on Conan O'Brien, Danny DeVito has stated that he's up for pretty much ANY storyline the show might throw at him save for one topic: prison rape. Apparently, DeVito informed series creator Rob McElhenney that was the only type of shock plotline that he refuses to have anything to do with.note Offhand jokes about prison rape are apparently fair game, though. In season six's "Mac's Mom Burns Her House Down", there's a joke about Frank thinking that prison rape happens at nursing homes.
The writers of Game of Thrones were such big fans of the show that they ended up writing "Flowers For Charlie".
Rob McElhenny also landed a small role on LOST after meeting co-creator Damon Lindelof, who is a big fan of the show.
Guillermo del Toro made a cameo appearance as the McPoyle family's insane patriarch in a season 8 episode. Apparently, while they were working on Pacific Rim, Del Toro mentioned being a big fan of the series to Charlie Day and he liked the rat-killing monologue so much that it was the reason he cast Day in the film.