Stacy Keach plays a Manipulative Bastard, politically incorrect older man who seems to love drinking, much like his role on Titus. His character's fondness for brutal, violent, politically incorrect, rule-bending and old-school law-enforcement techniques would also see him get on well with Mike Hammer.
Kyra Sedgewick plays a Deputy Chief sent to evaluate the 99, just like she played Deputy Chief Johnson in The Closer. She even has a rivalry with another straight-laced cop. At least she's using her actual accent this time.
Luis Moncada plays Tito Ruiz, the Giggle Pig kingpin, and is wearing the exact same suit he wore on Breaking Bad where he played Marco Salamanca, another menacing guy involved in the drug trade. As a bonus, Jake is very obviously dressed up like Jesse Pinkman in the same scene.
Terry is a talented artist. Terry Crews originally went to university on an art scholarship before earning a spot on his school's football team and going onto the NFL.
The Funeral guest stars Archie Panjabi as a member of the legal system who wants casual sex with a cop. Though this time she's a cop instead of a former investigator for the DA's office.
In Season 3's "Into the Woods", Gina suggests that Santiago change her first name to "Vanessa" since Gina considers it more attractive. “Vanessa” is the middle name of Gina’s actress, Chelsea Peretti.
Actor-Shared Background: In "The Apartment", Gina and Jake reference being friends since they were kids and hanging out together. Andy Samberg and Chelsea Peretti, who play Jake and Gina, have actually known each other since they were kids.
Terry Jeffords having daughters, owning a minivan and being an artist all came from the actor's background
Oddly enough, Gina getting Holt addicted to a cellphone game is based off a real game Chelsea Peretti got Andre Braugher hooked on
The scar on Rosa's right eyebrow is an actual scar from when Stephanie Beatriz tripped over a LEGO brick as a child.
Both Melissa Fumero and Amy Santiago are ethnically Cuban.
Casting Gag: This isn't the first time that Andre Braugher has played an uber-serious and imposing police officer. But unlike the grim and gritty environment of urban realism that Detective Frank Pembleton starred in, Captain Ray Holt operates in a comedic setting populated by goofy eccentrics. On top of that, Frank Pembleton was the brilliant-but-arrogant lone-wolf cop who refused to play with others and had to be pressured into doing so by his commanding officer. In Brooklyn Nine-Nine, Ray Holt is the commanding officer who has to deal with an arrogant lone-wolf detective who refuses to play with others and has to be pressured into doing so (Peralta). And to finish the gag off, on the earlier show Braugher played the partner of a detective who eventually came out as bisexual, while in this show he plays an openly gay police officer.
Dawson Casting: Inverted; while his age has never been specified Captain Holt is old enough to have been an active (albeit fairly young) detective in 1979. Andre Braugher, the actor who plays him, would have been 17 at the time.
Andy Samberg is older than his character by 3-4 years.
Executive Meddling: For "Operation: Broken Feather" Joe Lo Truglio was willing to perform a stunt where his arms would have caught fire but the studio stepped in and prohibited the show from setting the talent on fire. The finished episode uses CGI to show Charles being set ablaze.
Fandom Nod: Gina telling Rosa that in another lifetime they would have made a hot couple (and Rosa’s agreement with the statement) in Season 5’s “Game Night” has been seen as a nod to Gina/Rosa (or Dianetti) shippers in the B99 fandom.
Played with regarding Melissa Fumero's pregnancy in the third season, in which the producers seem to enjoy bouncing between covering it up with some occasionally ridiculous props, pretending that she's not visibly pregnant, and lampshading the hell out of it at every opportunity. By the last three episodes, they end up dealing with the situation by having Santiago go undercover in a prison... as a pregnant woman.
They went with the traditional methods for Chelsea Peretti's pregnancy in the fourth season by having Gina suddenly start wearing baggy sweaters and coats and tightly framing her from the chest up. Also, because Gina is supposed to be more than a little odd, she occasionally stands in weird positions to distract from her belly. Then the writers decided to have Gina suddenly announce that she was pregnant and in a relationship with a Boyle cousin!!!
I Knew It!: A lot of fans, particularly on Tumblr, guessed that Rosa is bisexual, and that the revelation about her personal life in 99 would be that she has a girlfriend. Naturally, they were delighted to be proven right.
Jossed: Of a sort; a popular fanfiction trope for the show post-Season 1 was to write a Hurt/Comfort Fic involving Jake ending up heavily traumatised following all kinds of horrible things that happen to him and he is forced to do while working for the Mafia (and often with Amy being called upon to comfort him in some way when he returns). Not only did Jake appear much less-than-traumatised when the Season 2 premiere finally rolled around, but a later episode revealed that Jake's time undercover was in fact quite mundane and boring, and mostly involved him doing low-level IT work.
Milestone Celebration: The 99th episode is treated as this, actually called "99." It involves the death and funeral of the Captain before Holt, the group going to LA and visiting Nakatomi Towers from Die Hard, getting stranded across the country when Holt needs to get back to be interviewed as a candidate for Police Commissioner. It even includes a big revelation about Rosa, she is bisexual.
Mean Character, Nice Actor: Stephanie Beatriz is much more cheerful and bubbly in real life than Rosa Diaz, whom she plays on the show. She also tends to wear much brighter clothing, in contrast to Rosa, whose wardrobe of choice is black leather. Beatriz's natural voice is also much higher-pitched.
Screwed by the Network: Season 4 was abruptly put on hiatus, on a major Cliffhanger no less, with Fox only saying that the show would return in the spring. When it did finally come back, it was to low ratings and the final six episodes were given a three week burn off. Fans were concerned about a potential cancellation but Fox finally decided to pick the show up for a fifth season.
Rosa's character was named Meghan during initial development of the show. Terry wasn't even planned to be a character, but after finding out Terry Crews was a fan of his work, Michael Schur based Sgt Jeffords on him.
Stephanie Beatriz actually had two auditions for the show, once for Amy, once for Rosa. She credits the first audition for allowing her to become comfortable and win the Rosa part.