As a person that works in a bank, I would do this in the following scenario:
As Bank employees, we have been taught to follow the criminals instructions to avoid any injuries or death of customers and employees. Most of the time a robber only wants to run in, get the most cash he/she/they can and get out as quickly as possible.
Often, a robbery that goes bad is because someone did not follow the instructions of the bank robber, or they tried to be a hero. If everyone does what they are told, generally, the crook grabs the money and gets the hell out.
I would only draw my weapon and take action if the robbery went into such a bad state of events that the robber or robbers, started shooting or hurting people. Or if I sensed that they were going to blow somone away 100 percent. I will not risk escalating the event to a more serious degree if I can. I do not want to possibly endanger my fellow workers and customers lives more than they already are.
As somone mentioned earlier there other factors to take into factor here. How do you know there is only one robber? I've seen the situation where one has cased the place while disguised as a customer while the other one does all the "robbing". Generally that person is the backup and is watching to see if anyone does anything "stupid"
In short, don't attempt to save the day unless you absoluteley, 100 percent, have to do something. You are best served to follow the instructions and not make yourself a hostile target for the criminals to lock onto.