His most famous role was as himself in the two-character film My Dinner with Andre[?], with Andre Gregory[?]. The two actors also wrote the script, which contrasted Shawn's modest down-to-earth humanism against the extravagant New-Age fantasies of Gregory, leaving the viewer of the film in an ironic suspension between the two viewpoints.
Other notable appearances include his role as the Masked Avenger in Allen's Radio Days[?] (1987) ("Beware, evildoers! Wherever you are!"), as the evil Vizzini in The Princess Bride (1987), and as Uncle Vanya in David Mamet's idiosyncratic Chekhov production, Vanya on 42d Street[?] (1994), a reading of the play set in a crumbling theater.
Shawn is a widely-used character actor on television, where he has appeared in many genres and series. He has had recurring roles as a Ferengi on the Deep Space 9 series of Star Trek, a comic ex-reporter on Murphy Brown[?], the Cosbys' neighbor on The Bill Cosby Show and on many other shows. He is also an accomplished voice actor, appearing especially in animation (including Toy Story and Toy Story 2 where he played "Rex the Green Dinosaur") and commercials.
Shawn's career spans all aspects of "low" and "high" culture, and his plays, unlike some of his television appearances, are considered very serious (even if they often have comic aspects). Among the best-known of these are Aunt Dan and Lemon and The Designated Mourner (1997), in both of which he appeared off-Broadway.