Essa linha foi improvisada por Jack Nicholson como uma imitação da introdução de Ed McMahon para o The Tonight Show.
De aqui :
Jack Nicholson ad-libbed the line "Here's Johnny!" in imitation of announcer Ed McMahon's famous introduction of Johnny Carson on U.S. network NBC-TV's long-running late night television program The Tonight Show Starring Johnny Carson. Stanley Kubrick, who had been living in England since before Carson took over "The Tonight Show," had no clue what "Here's Johnny!" meant. Carson once used the clip of Nicholson as the introduction to one of his annual anniversary specials.