Ray Liotta was the initial actor thrown around by the creators for the role of Frank Reynolds, but they immediately shifted gears after learning that Danny DeVito was interested. Suffice it to say, Frank (and the show in general) would have been very different if this happened, with Glenn Howerton speculating that Frank would have become the character that possessed a Hair-Trigger Temper rather than Dennis.
Aaron Paul was set to film a cameo at one point, but his filming schedule for the Need for Speed movie prevented this. It eventually came to fruition in the sixteenth season, which reunites him with his Breaking Bad co-star, Bryan Cranston.
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.
In one of their early attempts to film a pilot, Rob McElhenney was focused on filming and didn't plan to appear on camera at all. Instead, the Gang had a member named Rob played by David Hornsby (Cricket).
One of the earliest versions of the script that would eventually evolve into the show's pilot had Rob McElhenney and Charlie Day playing washed-up adult versions of Haley Joel Osment and Frankie Muniz, respectively. This was eventually referenced during the promotional campaign for Season 8, which depicted Osment becoming The Other Darrin for McElhenney's character.
Gail the Snail was originally written to have a brother named Not Quite Right Robin, with the idea being that they were the Garbage Pail Kids to Dennis and Dee's Cabbage Patch Kids. The character got phased out of the script when it was decided he was ultimately superfluous to the episode.
A Season 6 episode "The Gang Gets Successful" was written and filmed but went unaired. Its basic plot and scenes were adapted for the Season 7 episode "How Mac Got Fat".
A proposed Working Title for the show was Jerks, among many others. Glenn Howerton has stated that somewhere he still has the list of working titles they came up during development.
Dave Bautista auditioned for the role of Luther McDonald.
Rob McElhenney's first instinct when deciding on Mac's full name was Jimmy McDonald. He was only persuaded to change it to Ronald McDonald when he was told to make it more of a comedic reveal.
The Waitress was given a name in one of the pre-series pilots. While it is no longer considered to be the character's actual name in canon, the cast still refuse to disclose what it was for the sake of maintaining the joke.
A whole script was written for Season 5 but never used. It involved a member of the Jackass crew visiting the bar to drown his sorrows while the Gang tries to perform Jackass-esque pranks on him. It was suggested on the Sunny podcast that the crew might repurpose the script in the future.
The Waiter was intended to be revealed to be the same person as the HR Director seen in "Sweet Dee Has a Heart Attack" due to both characters being played by the same actor, but this has thus far never happened.
An idea floated for Season 15 was to have Cricket benefit heavily from the COVID pandemic. Cricket would run a successful business, and no one would know he was homeless because of virtual backgrounds and remote work.
A cut subplot for "The Gang Replaces Dee With a Monkey" had Cricket attempting to compete with the titular monkey for the spot of Dee's replacement.
Rob McElhenney wanted the others to gain weight for Season 7 along with him, but they all refused. Danny DeVito actually agreed to put on weight, but Rob rejected the idea out of fear for Danny’s health.
According to Kaitlin, RCG were trying to figure out a way to get Dennis and Dee to sleep together, but they couldn’t quite work it out and so Dee slept with Charlie instead. Lampshaded in “Time’s Up”, as Dennis hears she raped Charlie, and his main problem sounds like her choice of partners.
, 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.