In 1982, he took a writing assignment for CBS in Los Angeles. On the day he was scheduled to return to London, he auditioned for a role on the upcoming Cheers. The character of the postman did not exist when John auditioned for Cheers. When he finished auditioning for another part, he asked the writers if they had a bar "know-it-all" character.

Since they did not, John described such a character, "I explained to them that every neighborhood bar has a resident know-it-all and then demonstrated my version of him." An American icon was born in just five minutes of improv. John's improvisational skills created the lovable postman and trivia king, "Cliff Clavin." The Cheers team jumped on John's concept immediately, rewriting the pilot to include the quintessential mailman.

John's mastery improvisation as to what "Cliff" would say or do, did not stop at the audition. On the set, John was well known to re-write his own lines, always using the same theme and the same allotted time, so as not to upset the rhythm of the other actors. John is credited with the creation of "Cliff Clavin" on Cheers.

John's hugely successful 11-year role of Cliff achieved iconic status. In the history of TV, only a handful of shows have achieved the worldwide success of Cheers. During its first year, Cheers amassed 28 Emmys, and created legends of its stars and characters. To this day, Cliff Clavin continues as one of America's most loved characters through the daily syndication of Cheers.