Was almost canceled 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 14th scheduled season ends it will be tied for longest running live action american comedy series.
During a convention interview, star 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.
In the season finale of the first season 'Charlie Gets Molested' it is revealed that Charlie has a sister, however she is never mentioned by name or seen in the episode and is never once mentioned or referred to in any episode since.
The show was almost canceled 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.
In June 2011 it was reported that Rob McElhenney had gained 50 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 makeup 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."
Despite the reports that the show's pilot was shot for $200, it has actually been revealed that the pilot may have cost as little as $85 to shoot. The budget went almost solely to tapes for the camera.
Since Danny DeVito was only available for 20 days of shooting during season 2, the production crew had to work on all 10 episodes of the season at once, often shooting scenes from multiple episodes in one day.
In the episode "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.
According to creator Rob McElhenney, the name "Sweet Dee" was the nickname of one of his friend's girlfriends. He liked the nickname and later thought of it while creating the character of Dennis' sister.
The pilot was created by four Hollywood actors (Rob McElhenney, Glenn Howerton, Charlie Day and Jordan Reid) about struggling as actors in LA. The show was initially titled, "It's Always Sunny on TV". The original, unaired pilot only cost $200 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 LA to Philadelphia and re-shot the pilot.
In the episode "Sweet Dee Has a Heart Attack" (season 4), 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 starred.
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."
Season 6 included an episode titled "The Gang Gets Successful" that was never aired on FX. In Season 7, some of that footage was used for the episode It's Always Sunny in Philadelphia: How Mac Got Fat (2011). In addition to showing Rob McElhenney before his 50-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 10 minutes after it was proposed).
In an interview, star Kaitlin Olson said she broke her back during an accident in the summer of 2008. She was still recovering during the shooting of season 4, which caused an episode where Sweet Dee falls down a flight of stairs to be pushed back to season 5. Olson said she felt it wasn't a smart idea to do the stunt while recovering from a broken back.
In season 6 episode 3 (The Gang Buys a Boat), Dee mentions she wanted to throw a 'P-Diddy party', later in the series, season 8 episode 3 (Charlie's Mom Has Cancer), P-Diddy (Sean Combs) guest stars as the shady doctor to treat Charlie's Mom's cancer.
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.
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 dialect 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.
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.
The trivia items below may give away important plot points.
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, wherein he experiences an inability to socialize properly and often resorts to substance abuse (further evidenced by his alcoholism and his addiction to crack cocaine).
Mac actually has (had) two gay cousins. The first is Colin who appears 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 9th 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 Dennis Reynolds could be diagnosed with all these personality disorders Dennis Reynolds= Narcissistic Personality Disorder, Antisocial personality disorder (Psychopathy/Sociopathy), Borderline Personality Disorder. In fact in season 10 ep. 3 Psycho Pete Returns, Dennis is diagnosed with one of these.
In season 10 "The Gang Beats Boggs" Frank claims to be a doctor named "Mantis Toboggan" the same doctor that Dennis uses in "The D.E.N.N.I.S System" in season 5 to get pills for his grandmother so he can bang the pharmasist.