Sebastian Stan (born August 13, 1982) is a Romanian actor, best known for his role as James Buchanan "Bucky" Barnes / Winter Soldier in Captain America: The First Avenger and Captain America: The Winter Soldier. He will again reprise his role in the upcoming film Captain America: Civil War (2016). He played The Mad Hatter/Jefferson on the ABC's fantasy drama Once Upon a Time, Prince Jack Benjamin on Kings and Carter Baizen on Gossip Girl. He also appeared in Black Swan. In 2012, he appeared in the USA Network miniseries Political Animals starring alongside Sigourney Weaver.