Career Resurrection: For Michael Chiklis, whose career had fallen into a bit of a lull in the years after his first show The Commish had ended. Luckily, his wife convinced him that he should reinvent his image by working out and shaving his head to open up more opportunities which led directly to him being cast as Vic Mackey.
Executive Meddling: Episodes three and four were supposed to air in reverse order, but were ordered switched in order to continue the themed arc with Shane's coping with the aftermath of the murder of Terry Crowley. Also, the hiring of Glenn Close in season four was done after FX Network effectively gave notice to Shawn Ryan that they were seriously considering canceling the series after season four. Also, the haphazard splitting of season five and six (as it was filmed, the series was to have ended with Vic being notified that Claudette had found a replacement for him/Vic killing the man he thought responsible for Lem's death was done mainly due to FX constantly going back and forth on how many episodes season five and six would consist of and whether or not the show would have received a seventh season.
Reality Subtext: On numerous DVD commentaries, Shawn Ryan has stated that the character of Detective Dutch Wagenbach is largely based off of the real life/personality of Jay Karnes, the actor who plays the character. This has led to much teasing between Ryan and Karnes (who are long-time friends) on the DVD commentaries, whenever Ryan points out that just about every failed relationship the character Dutch has over the run the series is based (loosely) off of a real life failed relationship Karnes had.
Type Casting: Subverted; the series helped destroy the public image of Michael Chiklis as the stern but lovable father figure that had been hung around his neck since his early 90s series The Commish ended.
What Could Have Been: Vic Mackey was originally conceived as a veteran police officer in his late 50s, with Shawn Ryan namechecking the actor Harrison Ford as far as his original vision of what Vic would be like (in other words, Harrison Ford playing a villain). Claudette was a male character whose gender was flipped when they couldn't find a male actor to play the role.
Neither Ronnie or Lem existed in the original script; David Rees Snell and Kenny Johnson initially auditioned for the roles of Shane Vendrell and Terry Crowley, respectively, before Ryan created the roles of Ronnie and Lem for them.
Shane was supposed to have been killed by Vic during their confrontation midway through Season Four (after Shane was ordered to kill Vic by Antwon Mitchell). Lem's death in Season Five wasn't decided until midway through filming of that season; also dropped (but later resurrected as a foiler to keep Lem's death from being leaked out) was the notion of Shane being the one who delivered Danny's baby — and that Danny would die due to complications from the delivery.
The character of Shane may not have survived Season One if FX had gotten their way. The network didn't care for Walt Goggins and made it clear to Shawn Ryan that they might order Goggins fired at the end of the season. Ryan and the rest of the writers ended up crafting several episodes just to convince the suits to keep Goggins around, and it worked; they successfully saved his job and convinced the network that they were wrong to casually dismiss the actor's talents (which may be why Goggins was later cast as one of the lead actors in the FX series Justified).
Season Seven might have been quite different were it not for some behind-the-scenes scheduling conflicts:
The season was originally conceived around Franka Potente reprising her role as an Armenian crime boss; when she decided to do the film Che instead, the writers were forced to rewrite the plotlines involving the Armenian crime syndicate.
The second half of the season was impacted due to Walt Goggins appearing in the film Miracle at St. Anna; to allow Goggins to do both projects, Shane was turned into a wanted fugitive (which meant that they could film Goggins' scenes within a relative short time).
The Wrestling Stable
Fan Nickname: The Big Boss Men (due to their similar appearance to the Attitude Era Boss Man) and Nexus 2 (due to being former NXT wrestlers interfering with matches and causing mass beat downs.)
What Could Have Been: Ambrose was suppose to debut earlier through a feud with Mick Foley. For what ever reason it was dropped, and he debuted with The Shield.
In their debut, The Shield was supposed to come out with actual riot shields.