Was almost cancelled after its first season, but renewed at the last minute when FX executives decided to give it room to grow. It has gone from the brink of cancellation, to being one of the highest rated and most popular shows on FX, and when the fourteenth scheduled season ends, it will be tied for longest running live-action American comedy series.
In June 2011, it was reported that Rob McElhenney had gained fifty pounds in the previous six months. McElhenney has stated that he noticed characters from other sitcoms get better looking as the series progress (because the budgets get bigger, and that means more and better make-up people), but McElhenny wanted his character to do the opposite. Co-star Glenn Howerton started referring to the character Mac as "Fat Mac". When McElhenney asked if he could be called "Big Mac", Howerton said he would call him "Big, fat Mac".
During a convention interview, Glenn Howerton was asked why his character was named Dennis, whereas Charlie Day and Rob McElhenney's characters are named after themselves. Howerton responded by saying that he wanted to distance himself from his character as much as possible.
The show was almost cancelled during the first season of shooting. The President of the FX Network wanted to add a new character, and he was friends with Danny DeVito, so he decided to approach him to ask about joining. DeVito's children were big fans of the first season of the show, so he decided to sign on for ten episodes. DeVito enjoyed being part of the show so much, he became a regular member of the cast.
Since Danny DeVito was only available for twenty days of shooting during season two, the production crew had to work on all ten episodes of the season at once, often shooting scenes from multiple episodes in one day.
The pilot was created by four Hollywood actors (Rob McElhenney, Glenn Howerton, Charlie Day, and Jordan Reid) about struggling as actors in Los Angeles. The show was initially titled, "It's Always Sunny on TV". The original, unaired pilot only cost two hundred dollars to make, and was shot on a digital camera, with the actors taking turns holding the boom. It was then shopped around to different networks, and was picked up by FX. They then moved the setting of the show from Los Angeles to Philadelphia, and re-shot the pilot.
In season two, episode ten, "Dennis and Dee Get a New Dad", we can clearly see Charlie giving himself a tattoo, which is meant to say "Bad News" in his apartment, while he and Mac try to "get hard". They are, however, interrupted by Frank and Charlie never finishes his tattoo. His tattoo, which says "Bad New" is visible in every episode afterwards.
Despite the reports that the show's pilot was shot for two hundred dollars, it has actually been revealed that the pilot may have cost as little as eighty-five dollars to shoot. The budget went almost solely to tapes for the camera.
During the development of the series, the writers were offered five hundred dollars to come up with a title that the creators liked better than "It's Always Sunny in Philadelphia". The title remained unchanged.
In the first season finale, "Charlie Gets Molested", it is revealed that Charlie has a sister. However, she is never mentioned by name, nor seen in the episode, and is never once mentioned or referred to in any episode since.
In season four, episode ten, "Sweet Dee Has a Heart Attack", the mental health clinic where Frank (Danny DeVito) ends up, is a nod to the movie One Flew Over the Cuckoo's Nest (1975), in which DeVito appeared.
Each episode ends with the RCG production logo (which are the combined first initials of Rob, Charlie, and Glenn) and the actors pictured in hoodies with a sound clip played backwards: "for playing this forward." At the end of the first season finale "Charlie Gets Molested", the message had an echo: "Is it brown? Is it brown? Good work. Is it brown? Is it brown? Is it brown? Good work."
In an interview, Kaitlin Olson said she broke her back during an accident in the summer of 2008. She was still recovering during the shooting of season four, which caused an episode where Sweet Dee falls down a flight of stairs to be pushed back to season five. Olson said she felt it wasn't a smart idea to do the stunt while recovering from a broken back.
Season six included an episode titled "The Gang Gets Successful" that was never aired on FX. In season seven, some of that footage was used for episode ten, "How Mac Got Fat". In addition to showing Rob McElhenney before his fifty-pound weight gain, the story explained that the Gang had a Dee-conceived plan to hire individuals who looked like them as "avatars", so they could enjoy their newfound good fortune, but Mac's efforts to emulate his bodybuilder avatar went disastrously awry (and everyone else forgot the plan about ten minutes after it was proposed).
The set for Paddy's pub is also the set for MacLaren's Pub on "How I Met Your Mother" only stripped down and shot from different angles. The two shows also share street sets, most noticeably the famous NYC street featuring the downstairs entrance to MacLaren's Pub can be clearly seen in S12E6 Hero or Hate Crime
The original title for season two, episode five, "Hundred Dollar Baby", was scheduled to be "Sweet Dee Takes Steroids", but it was changed to be a play on words of the movie, Million Dollar Baby (2004).
The Philadelphia landmarks seen in the series' opening credits montage are the Benjamin Franklin Bridge, Penn's Landing on Market Street, Lincoln Financial Field, Swann Memorial Fountain in Logan Circle, Boathouse Row, 30th Street Station, and South Street Philadelphia.
Ben the soldier wears his Army-issue desert boots and "rigger's belt" mixed with civilian clothes off-duty. This is considered extremely dorky by actual soldiers, making it perfectly in character for him to do so.
In season six, episode three, "The Gang Buys a Boat", Dee mentions she wanted to throw a "P. Diddy party", later in the series, season eight, episode three, "Charlie's Mom Has Cancer", P. Diddy (Sean Combs) guest stars as the shady doctor to treat Charlie's mom's cancer.
Every installment of the Gang's "Lethal Weapon" sequels involves Dennis playing Martin Riggs (with a thick Australian accent, satirizing Mel Gibson's inability to maintain a consistent American accent during the early "Lethal" movies), and Mac as Roger Murtaugh (in full, and completely offensive, blackface). The two always switch characters halfway through. Both actors perform spot-on imitations of Danny Glover's voice.
Both Glenn Howerton (Dennis) and Rob McElhenney (Mac) had parts in the TV show ER. Rob played Andy a hurt firefighter and Glenn played Dr. Nick Cooper for 6 episodes. They have both also acted in Fargo and The Mindy Project as well.
Perhaps coincidentally, there is a Pennsylvania urban legend about "the Green Man", also known as "Charlie No-Face". The legend is actually about Raymond Robinson, who lost many of his facial features during an electrical accident as a child and lived thereafter as a recluse in western Pennsylvania.
On Charlie's green Army jacket, on his right breast pocket, there is a blue badge with wings on it. This patch is an AT or aviation electronics technician rating symbol for sailors in the Navy. It is worn on the dress uniforms and signifies your job, or rate, in the Navy.
Dennis Reynolds occasionally declares that he doesn't have feelings. In addition, he frequently becomes aggressive and manipulates women and the gang to suit his own desires. All of this would make him a good candidate for Antisocial Personality Disorder (a sociopath), wherein he experiences an inability to socialize properly, and often resorts to substance abuse (further evidenced by his alcoholism and his addiction to crack).
Mac actually has (had) two gay cousins. The first is Colin, who appeared in the first episode of the series, "The Gang Gets Racist", when the gang is running the biggest "gay bar" in Philadelphia. The second gay cousin is in the ninth season, "Mac Day". Country Mac is gay, but ends up dying from a motorcycle accident by the end of the episode.
According to many psychologists and psychiatrists, Dennis Reynolds could be diagnosed with all of these personality disorders: Dennis Reynolds= Narcissistic Personality Disorder, Antisocial Personality Disorder (psychopathy/sociopathy), and Borderline Personality Disorder. In fact, in season ten, episode three, "Psycho Pete Returns", Dennis is diagnosed with one of these.
In the season ten opener, "The Gang Beats Boggs", Frank claims to be a doctor named "Mantis Toboggan", the same doctor that he uses in season five, episode ten, "D.E.N.N.I.S. System", to attempt to seduce a pharmacist.