Dylan O'Brien pulls double duty as Stiles and the Nogitsune, when they are presented as separate entities.
Aaron Hendry plays the Nogitsune, or rather one particular nightmarish variety of him, and Brunski, one of Eichen House's orderlies.
Crystal Reed played Allison Argent for three seasons, and returned for one episode as the first Hunter and the original Argent matriarch, Marie-Jeanne Valet.
Dawson Casting: Although not as egregious as the main cast of some other shows set in high school, the only actor in the main cast who portrayed a character his own age in the beginning was Tyler Hoechlin, who was 23 when he started as Derek. It's only gotten worse, as the show advances only about a quarter as fast as real life. At the time they first started portraying 16- and 17-year-olds, the actors were:
18: Gage Golightly (Erica), the only one who was of normal high school age at the time of filming.
19: Tyler Posey (Scott), Dylan O'Brien (Stiles)
22: Colton Haynes (Jackson), Stephen Lunsford (Matt)
24: Holland Roden (Lydia), Keahu Kahuanui (Danny), Charlie and Max Carver (Ethan and Aiden)
Playing Against Type: In an interesting variant, it's the fact that Doug Jonesnever dons any prosthetics during his appearance as Barrow. Which is pretty shocking considering not only his penchant for doing so in other works and the prevalence of such effects on this show.
Linden Ashby, who plays Sheriff Stilinski, is married to Susan Walters, who plays Lydia's mother Natalie.
Dr. Fenris, a character who first appeared on a web series spinoff and in Season 4 also appears on the show, is played by John Posey, Tyler Posey's father. John Posey had also played another doctor on Teen Wolf in Season 1.
Kira was originally intented to remain on the show for its final season with Arden Cho saying at one point that she was looking forward to return before either MTV or Jeff Davis decided to write her out from the show, which given the fan backlash on how she departed and how she was a character who had good potential but was wasted on the show, is partly why she was the one of the notable characters who didn't come Back for the Finale like Derek and Jackson and got a near-Chuck Cunningham Syndrome treatment as she was mentioned once in the final season.
Word of God: Although it's never been specified in-show how Scott and Stiles became friends, Tyler Posey (Scott) and Dylan O'Brien (Stiles) have said that they met in the sandbox when they were four, Stiles peed on Scott's sandcastle and that was that. O'Brien also uses this to explain their Only Friend problem: From then on they just decided there was no need to hang out with other people.