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According to the site Looper, shortly before the beginning of season 6, cast members Paget Brewster (Special Agent Emily Prentiss) and A.J. Cook were fired from the show. The official word from the network and production company came down to budget cuts, but according to Brewster, the real reason was far seedier. Reportedly, someone from CBS called Criminal Minds showrunner Edward Allen Bernero and ordered him to bring in "new women"--meaning younger women. Cook shot an episode to explain why her character, J.J., was going away, but Brewster's agent negotiated an additional 17 episodes before her demise. Actress Rachel Nichols was brought in to replace Brewster and Cook, and Brewster went on to shoot a pilot. Once the pilot wasn't picked up, Criminal Minds producers (new ones; Bernero quit) claimed that those 17 episodes constituted another year of her contract, forcing her to return to the show that had fired her (and killed off her character). After thousands of fans wrote in letters of complaint, Brewster and Cook returned to Criminal Minds as well.
Most of the playful banter between Derek Morgan (Shemar Moore) and Penelope Garcia (Kirsten Vangsness) stemmed from the way the actor and actress talked to each other in real life.
Matthew Gray Gubler does not wear matching socks, because he thinks it's unlucky. In a few of the episodes, you can see his mismatched socks.
The entire cast have mentioned that they are like one big family. Joe Mantegna calls Shemar Moore and Matthew Gray Gubler his "golden boys".
David Rossi has mentioned he knows Sir Ringo Starr. There is even a picture of him and Starr on Rossi's desk. Joe Mantegna and Sir Ringo Starr are friends in real life.
The character "Penelope Garcia" was supposed to be in only one episode, but she tested so positively that she was added to the main cast, and has the highest appearance count appearing in all but one episode (Season 1 Episode 5; "Broken Mirror").