What would you do if you witnessed a fight break out?

(This is for a scene in my fiction. Since I am writing realistic fiction, I just want to know if my characters’ reactions were realistic.)

Let’s say that you were in a public place, let’s say a school or a mall or some place (not a dark alley). Two people that you have met before but who are not your friends suddenly start fighting. You have no idea why they are fighting. All you know is that both of them have a reputation for being mean to others.

Seeing the fight start, what would you do? Would you call 911? Try to break up the fight? Walk away because it is none of your business? Something else?