I'll never forget the first Collegiate Chorale rehearsal I attended. Rob (or Berg, as he is affectionately known to his high school students) conducted the entire rehearsal in silence. He used gesticulations, exaggerated facial expressions, the piano, and the chalkboard to get his point across.
Since Rob, with his expressive features and tireless enthusiasm, gives the impression of a cartoon character come to life this was amusing, unsettling, and highly effective. And clearly memorable, since it's been well over a decade since that day and I can recall it in such detail.