Greenwood is a Canadian actor, best known in the US for his roles in I, Robot, The Core and Thirteen Days (in which he played President John F. Kennedy) and Capote (in which he played Jack Dunphy, Truman Capote's lover). On television, Greenwood has had roles on St. Elsewhere (Dr. Seth Griffin, 1986-1988), Knots Landing (Pierce Lawton, 1991-1992) and the UPN series Nowhere Man (Thomas Veil, 1995-1996).