So... the people listed below have been in both the Buffyverse and Criminal Minds verse. I'm pretty sure this is
not every one, particularly as Criminal Minds is ongoing, and I have only seen all of the first 15 seasons and the two spinoffs. There's an additional 4 seasons (Criminal Minds Evolution) and at least one more to come.
These cast members are mostly who i recognized and remembered - 27 total.
In addition to the many cast members in common, there was an episode of CM that was an homage to the Buffyverse (season 10, episode 7, 'Hashtag') (see below for more on the episode). Nearly all the guest characters are named for Buffyverse characters. I get the impression that one or more of the production staff of CM are huge fans of the Buffyverse.
I didn't want to try to make a table so the order is:
Actor - Buffyverse/Whedonverse character -- Criminal Minds character, maybe some details.
Nick Brendon - Xander Harris -- FBI technical analyst Kevin Lynch for multiple seasons. Was love interest for the primary technical analyst Penelope Garcia for much of that.
Michelle Trachtenberg - Dawn Summers -- Diane Turner, a crazy, scary unsub (unknown subject) -- One of the most disturbing and tragic episodes of the entire Criminal Minds series.
Juliet Landau - Drusilla -- Catherine Heathridge, A scary, creepy Gothic Criminal Minds episode called Heathridge Manor
Bailey Chase - Graham Miller -- Jason Battle/James Baylor - A murderous and scary unsub who went after Penelope
Leonard Roberts - Forrest Gates -- Kaman Scott, a tragic arsonist who lost his mind
Gary Grubbs - Roger Burkle - a cop -- Detective Oren Carr
Lindsay Crouse - Dr Maggie Walsh -- Mary Mays, A narcissistic, snobbish parent of a mentally ill unsub who was way worse than the unsub himself.
Gina Torres - Jasmine on Angel and was also Zoe in Firefly) -- Detective Thea Salinas
Glenn Morshower - Cassie Newton's father - Lt Kevin Mitchell
Jack Conley - Cain, Werewolf hunter (BtVS), Sahjhan (Angel, recurring) -- John Summers - hero-victim, CIA agent/protector
Nick Chinlund - Colonel Ellis post initiative initiative - Max Weston CM, Rawlins, CM suspect Behaviour
Bridgid Brannagh - Wesley's scary/funny first girlfriend on Angel (Guise will be Guys) -- Monica Kingston, victim/survivor, mother of victim
Harry Groener - The Mayor -- Leonard Hagland
Abraham Benrubi - Olaf the Troll -- Frank Fielding
Sam Anderson - Holland Manners Wolfram and Hart lawyer, Angel -- Dr Rawlings
Jeff Kober - Kralik (Helpless), Rack (recurring season six) -- Leo, (alt personality of unsub) in episode 'derailed'
Mercedes Mcnab - Harmony (BtVS recurring and Angel main character) -- Survivor (not a victim) of a demented unsub
Riki Lindholme - Cheryl (Him) -- Vanessa Holden
Carmine Giovinazzo - Buffyverse's very first victim, aka the boy in the locker -- Andrew Meeks
Keith Szarabajka - Daniel Holtz, Angel (recurring victim of Angelus, and villain) -- Detective Hardy Friedman
Aimie Garcia - Cynthia (Angel) - Dr Julia Ochoa
John Rubinstein - Linwood Murrow, Wolfram and Hart lawyer, Angel -- Hayden Rawlings
Conor O’Farrell - Detective Stein -- Detective Craw
Jack Plotnik - Deputy Mayor Allan Finch -- Tanner Johnson
Jeremy Rachford - vampire Lyle Gorch -- Boris Poshakov on Criminal Minds, Beyond Borders
__________Not in Buffy or Angel series_______
Tahmoh Pennikett - Paul Ballard, Dollhouse -- Michael Hastings
Luke Perry Pike Buffy the Vampire Slayer movie, -- Benjamin Cyrus, Criminal Minds unsub (seriously scary preacher guy)
(there are probably more non buffyverse actors who were in other whedonverse series but I don't know them all, sorry)
😎 Criminal Minds speak: unsub = unknown subject -- the suspect
______________special criminal minds tribute to buffy episode_________________
the guest cast for the Criminal Minds Episode Hashtag. Note the names of most of them. (some aren't buffyverse, b/c they are recurring guest stars, but most of the cast's character names are some combination of a Buffyverse character name (they mixed up names for most of them):
The High School is 'Sunnydale'. Mascot is Razorbacks.