The newly greenlit project, from "Green Lantern" screenwriters Greg Berlanti and Marc Guggenheim and "Fringe" co-executive producer Andrew Kreisberg, has already lined up Stephen Amall ("Hung") as young Oliver Queen, David Ramsey ("Dexter") as Oliver's bodyguard John Diggle, and Susanna Thompson ("Once and Again") as Oliver's mother Moira.
Described as as "very provocative and sophisticated and edgy," "Arrow" centers on wealthy bad boy Oliver Queen, who after spending five years shipwrecked on an island in the South China Sea, returns to Star City with a mastery of the bow and a determination to make a difference.
Cassidy, who appeared as the manipulative demon Ruby in Season 3 of "Supernatural," will play Laurel -- the alter ego of DC's Black Canary -- a crusading legal aid attorney who shares a romantic past with Oliver. According to a leaked character breakdown, she's waging a one-woman war against the 1 percent following the death of her younger sister aboard Oliver's yacht.
No stranger to The CW, Cassidy also starred on the update of "Melrose Place" and had a recurring role on "Gossip Girl."