Step One -- Don't get caught flat-footed in the first place! Don't face the cashier directly at the register, quarter yourself so you have a view out the front window. If the front windows of a particular store are covered in advertisements, maybe consider shopping somewhere else, as that's simply begging to be robbed.

Step Two -- Get some local advice

As a former convenience store clerk, I would submit that if someone were pointing a gun at me, the last thing I would want--and this is key--is for someone to startle him!

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Personally, this falls into the ventilate-without-warning category. Because there are two options. Option One is that I truly believe in my heart of hearts that the attacker is going to shoot the clerk, at which point he's probably going to shoot me. Since my belief is that lives are in immediate and unavoidable danger, then giving warning is inconsistent. I should use every available advantage and tactic to ensure survival.

Option Two is that I don't believe the guy is really about to shoot someone, at which point warning is consistent, but I've just escalated a situation to immediate-deadly when I didn't believe it was there yet.

My considerations:

(1) I have no sworn duty, and my primary ethical responsibility isn't to the clerk (sorry, pal).

(2) As with any gunfight, you can lose, no matter how advantaged you are.

(3) The attacker you see may not be the only one present. He may have an accomplice already in the store, and he may have an accomplice outside, potentially with even heavier artillery.

So I would intervene only if the danger was truly imminent. Ideally when the muzzle isn't covering anybody. And I would absolutely act if he told everyone to get in the back room or something, as that's a prelude to execution.