The actor Joe Mantegna, who plays "David Rossi" on the show, is a close friend of the former drummer of "The Beatles", Ringo Starr, in real life. That is why there is always a framed picture with him and Ringo on "Rossi's" desk.
Matthew Gray Gubler, whom the fans mostly know as "Doctor Spencer Reid", always(!) wears two different-colored socks. The actor believes that socks of the same color are bad luck and he does not even want to take a chance while he is shooting Criminal Minds. Sometimes, you can even see a glimpse of his different-colored socks while watching a scene on the show...
All cases being solved by the special agents on Criminal Minds are actual real-life cases. Some aspects of the case are - of course - altered for the show, but the stories are all based on real events. The production company employs one person, whose sole responsibility is to look for real-life cases that might be used for a new episode.
James T. Clemente is one of the show's writers and he actually used to be a profiler for the FBI. Most of the episodes he authors are about crimes against children, which is what he mostly had to deal with while he was working for the Bureau.
"Dr. Spencer Reid" is quite a particular character on Criminal Minds. He was gifted already as a child and graduated high school when he was only twelve years old, but he also has many rather peculiar characteristics. Maybe it is because his mother was a schizophrenic?
Rick Dunkle, one of the show's producers wanted to have a little cameo on Criminal Minds just once, but also did not want to put too much effort when it came to the acting. So, he decided he would appear on the show as a dead man. Unfortunately for Dunkle, who is a huge Star Trek fan, he is actually murdered by the former Star Trek cast member Wil Wheaton, who plays the killer in the episode...
Mandy Patinkin (center), who played "Jason Gideon" and Thomas Gibson (right), also known as "Aaron Hotchner" weren't co-stars for the first time when they started out on Criminal Minds in 2005. As it turns out, they acted side by side once before, when they starred together in Chicago Hope for the first two seasons of the medical drama!
The name of Mandy Patinkin's character on Criminal Minds, ("Jason Gideon") was actually not picked randomly. The 65-year-old actor has a real-life son who's also named Gideon.
At the beginning of the fifth season, "Dr. Spencer Reid" is shot in the leg. This injury was actually not in the original script but it had to be added after the Matthew Gray Gubler hurt his knee in an off-set accident and could barely walk for four months!
In the opening credits for Criminal Minds, we don't only see the actors in their respective roles, but also pictures from case files of real-life criminals!