My big claim to fame during the three years I was at Stratford was understudying Henry V and going on without any rehearsal — and I tell that story in the show. … Tyrone Guthrie, a great English director of that time, said to me — I was understudying Chris Plummer — and they said, "Plummer's ill. Can you go on?" And I had never rehearsed the part, never spoken the part out loud. And I went on.
Interview 18 Feb 2012.
William Shatner Interview: It's 'Shatner's World' And He Wants You To See It : NPR