Paul Wesley is portraying Stefan, one of the three central protagonists. In Season 4, it's revealed Stefan's ancestor is Silas and he is Silas' doppelganger. It's revealed in Season Five, there is one other doppelganger of Silas' alive in the present day, Tom Avery, an innocent paramedic.
Acting for four in Nina Dobrev's case. She portrays Elena the protagonist, also Katherine a villain and one of the major antagonists. In Season Five Nina Dobrev also portrays Amara, the original Petrova, and Elena and Katherine are her doppelgangers. On the spin-off , she portrays Tatia in flashbacks to the 10th century and Katherine' is her doppelganger.
Mia Kirshner, who plays Isobel Flemming, Elena's real birth mother, played the role of a woman named Catherine in Not Another Teen Movie (granted, spelled differently from our Katherine).
Dawson Casting: The mostly teenage cast is played by actors in their 20s, making some casting choices downright bizarre. The actors who play the Gilberts are only one year apart, starting with Nina Dobrev (20), Steven R. Mc Queen (21), and Sarah Canning (22). And to top it off, Whitley Gilbert (48) a grandma?!
This is made extra weird by the casting of John Anders (born 1981) as Elena's father, John Gilbert. Isobel, who is supposed to be younger than him even without being a vampire, is played 35-year-old Mia Kirshner.
Justified in the case of Jasmine Guy as "Grams" Sheila Bennett; she's meant to have preternatural longevity as a witch. Stefan mentions first meeting her at a protest rally in the 60s, though he's not sure she would remember. She did.
Originally, the 4th-season-spanning cure storyline was supposed to resolve with Damon being the one to receive the cure. However, almost immediately after introducing the cure as an idea, the writers were dismayed to find that a large number of fans were predicting that very outcome. So they changed mid-course, and the cure was instead given to Katherine.
A deleted scene reveals Kelly Donovan, Matt's mother, as a succubus.
Nina Dobrev was only able to come back for the series finale, forcing the second half of Season 8 to be heavily rewritten to not have Elena (or Katherine) come back until then.