Nathan Parsons who plays the werewolf Jackson, left the show during season one to play a vampire on HBO's True Blood (2008) only to return to his werewolf character in season two after the series ending of "True Blood".
Steven Krueger (Josh), Claire Holt (Rebekah), Nishi Munshi (Gia), Meg Foster (Josephine), Nathaniel Buzolic (Kol), and Shane Coffey (Tim) all came out on at least one episode in the ABC Family tv show Pretty Little Liars.
Only one of the Mikaelson children is American Riley Voelkel. Joseph Morgan is English, Claire Holt and Nathaniel Buzolic are Australian, Daniel Gillies is from Canada and Casper Zafer is an Ukrainian-British actor. Devon Allowitz is also an American, but he's excluded since his character Henrik has only appeared in archive footage in the series.
The trivia items below may give away important plot points.
In an interview Leah Pipes (Camille O'Connell) stated that in one take of Klaus and Cami's second bar scene in 1x18, Joseph Morgan got so into the scene that he kissed her on the mouth, which took her by surprise.