Slowly I Turned

"Slowly I Turned" is a popular vaudeville sketch wherein a character is relating a story and is triggered into violent outburts when the listener inadvertently utters a triggering phrase. Versions have also been performed in cinema and on television. Comedians Harry Steppe, Joey Faye, and Samuel Goldman each laid claim to this routine, also referred to as "The Stranger with a Kind Face" by clowns, "Niagara Falls" by fans of The Three Stooges and Abbott and Costello, "Martha" by fans of I Love Lucy, "Pokomoko", and "Bagel Street".

The routine features a man recounting the day he took his revenge on his enemy - and becoming so engrossed in his own tale that he attacks the innocent listener to whom he is speaking. The attacker comes to his senses, only to go berserk again when the listener says something that triggers the old memory again.

Typically, the routine has two characters meeting for the first time, with one of them becoming highly agitated over the utterance of particular words. Names and cities (such as Niagara Falls) have been used as the trigger, which then sends the unbalanced person into a state of mania; the implication is that the words have an unpleasant association in the character's past. While the other character merely acts bewildered, the crazed character relives the incident, uttering the words, "Slowly I turned...step by step...inch by inch...," as he approaches the stunned onlooker. Reacting as if this stranger is the object of his rage, the angry character begins hitting or strangling him, until the screams of the victim shake him out of his delusion. The character then apologizes, admitting his irrational reaction to the mention of those certain words. This follows with the victim innocently repeating the words, sparking the insane reaction all over again. This pattern is repeated in various forms, sometimes with the entrance of a third actor, uninformed as to the situation. This third person predictably ends up mentioning the words and setting off the manic character, but with the twist that the second character, not this new third person, is still the recipient of the violence.

...
Wikipedia