Williamson was known for several tantrums and onstage antics. During the Philadelphia tryout of "Inadmissible Evidence", a play in which he delivered a performance that would win him a Tony Award nomination in 1965, he hit the equally mercurial producer David Merrick. In 1968, he apologised to the audience for his performance one night while playing Hamlet and then walked off the stage, announcing he was retiring. In the early 1970s, Williamson left "The Dick Cavett Show" prior to a scheduled appearance, leaving the host and guest Nora Ephron to fill the remaining time. In 1976, he slapped actor Jim Litten during the curtain call for the Broadway musical "Rex". In 1991, he hit co-star Evan Handler on the backside with a sword during a Broadway performance of "I Hate Hamlet".