For me, if I see an assailant with a weapon threatening lives, I will draw and fire. That means a punk with a knife or gun telling a cashier to open the drawer, a guy with a knife holding some woman down, a group of punks with bats pushing/beating someone around. The drawing and apparent use of a weapon means lives are at risk. A criminal with a weapon is an automatic declaration by said criminal that he intends to use it if he doesn't get what he wants:money, sex, respect, etc. Non-weapon beatdowns also qualify.
Any other situation where I see probable bad guys hassling good guys(but without a weapon or actual violence) will be met by a loud voice(mine), a cell phone call to the police, and probably some OC. I've set my limit, now it's up to them to either escalate or flee. If they flee, so be it. If they escalate to violence or weapons, that brings us back to the first part of the post.
If it's threatening me with a weapon, I will comply to their wishes but only in a way that can put me at an advantage, ie: throwing my wallet so they have to move to get it/take their eyes off me. Then I respond with my weapon. If it's someone hassling me without violence/no weapon, then a healthy dose of non-threatening verbal persuasion/fleeing/calling the police is the way to go. So if it's some irate driver after we just got into a car accident who's talking ####, I'll speak calmly, put my hand clasped in front of me in a non threatening but ready stance, and call them "sir" a lot.