Even though it's not technically your job to break up fights, if you don't take action to ensure the safety of the customers, there could be even more trouble.
You won't want to answer with an "always" statement because it depends on the situation and the protocol of the establishments which you have worked. Assure the bar manager that you are proactive and take action to secure the area.
"I understand that every bar will have their own set of rules when it comes to employees jumping into fights that break out. What is your policy here?"
"I had broken up my share of fights when I was a bouncer at a nightclub. I am capable of controlling a fight; however, I prefer that the current bouncers are left the room to do their job."