In the 1960s and early 1970s, he appeared in two productions of the New York Shakespeare Festival — Coriolanus at Central Park's Delacorte Theatre (1965), and Václav Havel's The Memorandum — and the David Newbburge-Jacques Urbont musical Stag Movie (1971), in which stars Sullivan, as Rip Cord, and Adrienne Barbeau, as Cookie Kovac, were "quite jolly and deserve to be congratulated on the lack of embarrassment they show when, on occasion, they have to wander around stark naked. They may not be sexy but they certainly keep cheerful", wrote The New York Times theater critic Clive Barnes in an otherwise negative review.