Shawn's involvement with theater began in 1970 when he met Andre Gregory, who has since directed several of his plays. As a stage actor, he has appeared mostly in his own plays and other projects with Gregory. He made his film debut in 1979, playing Diane Keaton's ex-husband in Woody Allen's "Manhattan" and an insurance agent in Bob Fosse's "All That Jazz". His best-known film roles include Earl in "Strange Invaders" (1983) and Mr. Hall in "Clueless" (1995). After seeing his performance in "My Dinner With Andre" (1981), casting director Janet Hirshenson was so fond of his delivery of the word "inconceivable" that she cast him as Vizzini in "The Princess Bride" (1987). Other roles include Baron Von Westphalen in "Southland Tales", Cyrus Rose on "Gossip Girl", and Ezra in "The Haunted Mansion" (2003).