"Hey now! Stop that" is probably a better answer than i could come up with. There is not enough information in the question to tell me exactly what i would (or should) do. Doing nothing would be irresponsible, but so might intervention. The last time this happened to me, i was in charge of the place where the fight occurred. One person was angry and drunk and challenging several people; the other was trying, unsuccessfully, to calm him down. I did not know what to do but felt i had to do something. I get between the two men and put my hand gently on the aggressor's shoulder. He turned his attention toward me and i guided him slowly out into the parking lot. He had been yelling about racism so i started talking about his heritage, etc, After about ten minutes his wife and friends came to gather him up. Was i right or just lucky? I'll never know.