Over its long 15-year history, the original CBS Criminal Minds has aired a whopping 324 episodes.
Thanks to the 'unsub of the week' format and the always intriguing theme of delving into the psychology of serial killers, the show hasn't lost its appeal, and in some ways, has become even more compelling.
Another element that attracts the hordes of fans to Criminal Minds is the plethora of guest stars who play the creepiest and most twisted unsubs and their victims, and some of these names leave both newcomers and seasoned viewers open-mouthed, making binge-watching an exciting hunt for familiar faces.
Here are five Criminal Minds cameos you might be shocked to remember.
Amber Heard as Lila Archer
S1E18 Somebody's Watching
Remember the Hollywood starlet who shared a pool kiss with Spencer Reid? That was Amber Heard. Her character, Lila, was the victim of a stalker who killed off potential threats to her career.
Years after guest starring on Criminal Minds, Heard became famous not only for her acting career, but also for her high-profile defamation trial against Johnny Depp. So when you re-watch Season 1 today, the appearance of the scandalous figure on screen may very well stop you in your tracks.
Mark Hamill as John Curtis
S8E24 The Replicator
The Star Wars legend played a big role in Criminal Minds Season 8 as the season's main antagonist, John Curtis, aka The Replicator. The former FBI agent and highly intelligent narcissistic serial killer blamed the BAU for his professional downfall and killed his victims in the style of previous unsubs.
Hamill appeared uncredited and unseen in many episodes before the direct confrontation between him and the BAU in the Season 8 finale. Even then, many viewers didn't recognize the character as Luke Skywalker.
Brian Baumgartner as Bill Harding
S9E22 Fatal
Probably the most shocking guest star appearance is that of Brian Baumgartner, who played sweet, dumb Kevin on The Office. You can't imagine two more contrasting characters than Kevin and Bill Harding, the serial poisoner.
Bill's story was heartbreaking. After a tragic incident in his childhood, he became obsessed with Greek mythology and had a dream to go to Greece. But that dream was never to come true, as Bill was diagnosed with terminal cancer. That's when he snapped and started poisoning the food of the people he was jealous of.
Tim Curry as Billy Flynn
S5E23 Our Darkest Hour & S6E1 The Longest Night
Billy Flynn, aka The Prince of Darkness, was without a doubt one of the most vile Criminal Minds unsubs. He had been robbing, sexually assaulting, torturing and killing people for 26 years before his and the BAU's paths crossed.
In the Season 5 finale, Flynn uses blackouts to stalk his victims (hence the nickname) and kidnaps a daughter of his previous surviving victim, trying to make her his accomplice. It takes the BAU two full episodes to take him down, and for viewers to get used to the idea that the unsub is played by Tim Curry.
Jason Alexander as Henry Grace
S4E8 Masterpiece
It is almost impossible to recognize the actor who played Seinfeld's George Costanza in the narcissistic psychopath determined to take revenge on Agent Rossi for putting his brother in prison.
The unsub is obsessed with math and sequences, which eventually leads Reid to find a key to his murders and kidnappings, and allows the BAU to outsmart him. But the mind games in this episode make it one of the most memorable in the series.