Hydra kept him alive in secret and turned him into the deadly assassin known as The Winter Soldier, and he’s factored prominently into Captain America: The Winter Soldier and Captain America: Civil War.
Sebastian Stan played a major role in kicking off Phase 3 and he’s just begun work on the conclusion, Avengers: Infinity War.
Stan has gone through a series of transformations since he made his debut as Bucky Barnes back in Captain America: The First Avenger.
The sheer fact that Stan is involved in Infinity War is not a surprise as he was previously confirmed to appear last August.