The Coen brothers have a knack for elevating character actors to leading men. They did it with actor Tim Blake Nelson in “O Brother, Where Art Thou?,” where he played Delmar O'Donnell. They did it with him again in “The Ballad of Buster Scruggs,” where he plays the title character. His story combines two Western tropes, the singing cowboy and the bloody, cruel Wild West. “The Ballad of Buster Scruggs” is nominated for a few Oscars this year, including Best Adapted Screenplay and Best Costume Design.