Also consider that even as an off-duty LEO, you would be taking some mighty big risks in intervening. Responding uniformed officers will not be able to immediately tell the good guys from the bad guys, and anything you do may get you shot, including nothing.
In the case of a robbery in progress, I'd call for back up, take a position of advantage behind whatever cover was available, and either draw or prepare to draw discretely. Stay on the line with the dispatcher, give them your location, your description, the BG's description, etc. Above all be on the lookout for companions in crime. I would not intervene unless I thought there was going to be an immediate use of deadly force by the BG. You gotta understand, as the good guys, we have to care where every bullet fired goes, including the ones the BG might fire. If the situation can be resolved without shooting, that's my first choice, even as an LEO. It should be yours also.