Sebastian Stan (born August 13, 1982) is a Romanian-American actor, known for his role as James Buchanan "Bucky" Barnes/Winter Soldier in the Marvel Cinematic Universe. He also appeared in the 2010 film Black Swan.On television, Stan portrayed Carter Baizen on Gossip Girl, Prince Jack Benjamin on Kings, The Mad Hatter/Jefferson on Once Upon a Time, and T.J." Hammond on the miniseries Political Animals.