Here is where I differ from BikerRN on these kinds of scenarios.
Now bear in mind, this is only if I am at "contact distance" from the BG and he is focused solely on the teller. (Which, in all honestly, I find this to be a highly unlikely situation.) But, if he's directly in front of me and doesn't know I'm there, I'm shooting him in the base of his skull about 2 inches from contact.
The reason is, is that in just a matter of moments he is going to be aware that I am that close and I don't want to be in that close a proximity to him when he starts waiving his gun around. I'm also not going to risk being the first one to be shot by being that close.
I also feel that being that close and trying to slowly sneak away really isn't going to work without being noticed and then it's anyone's guess as to how he is going to react.
Now, if I'm at any other location in the bank and not standing right next to him, then I'm back in BikerRN's camp and will try and make myself invisible and react accordingly if he actually starts shooting.
Also, nothing is set in stone... These situations are highly fluid and dynamic. I have a general idea on what to do in various scenarios and situations, but you really have to be there and respond to each situation on an individual basis as it plays out.
Now I'm no hero and I almost exclusively plan to keeping my head down behind cover and observing the situation and being a good witness. I fully understand most armed robberies go off without anyone being shot. I've viewed hundreds of closed circuit surveillance video's of armed robberies in progress. Whether it's a convenience store, bank, gas station, whatever... I know most robberies go off without people getting shot. But there are also those that don't happen without people getting shot or killed, even after complying. And I've seen those video tapes too. Plenty of them.
So, in the extremely rare and unlikely situation when a BG is standing directly in front of me a 12 or 18 inches away, I'm putting his lights out with a shot to the medulla.