The actor Richard Burton famously played Hamlet at the Old Vic in 1953.    The following story is from a profile of Burton written by journalist John McPhee in 1963 for Time Magazine, and recounted in the current New Yorker  (“Elicitation”, 7 April 2014, p.57):

He [Burton] had completed about 60 performances and the box office was beginning to slide when the house manager came to his dressing room one evening and said, “Be especially good tonight.  The old man’s out front.”
“What old man?”
“He comes once a year,” said the house manager. “He stays for one act and he leaves.”
“For God’s sake, what old man?”
“Churchill.”
As Burton spoke his first line – “A little more than kin, and less than kind” – he was startled to hear deep identical mutterings from the front row.  Churchill continued to follow him line for line, a dramaturgical beagle, his face a thunderhead when something had been cut.  “I tried to shake him off,” remembers Burton.  “I went fast and I went slow, but he was right there.”  Churchill was right there to the end, in fact, when Burton took 18 curtain calls and Churchill told a reporter that “it was as exciting and virile a performance of Hamlet as I can remember.”  Years later, when Winston Churchill – The Valiant Years was under preparation for television, its producers asked Sir Winston who he thought should do the voice of Churchill.  “Get that boy from the Old Vic,” said the old man.
They got that boy from the Old Vic.