Situation: You walk out of a gas station restroom to see one individual with a pistol at the register aiming it at the cashier. Neither person is aware you are there but you are carrying a concealed firearm. What is your response?

Simple, I stay hidden and stay silent. I reach into my cell phone and immediately silence the ringer and disable the vibrate notification. Next, I have two heroic options and one alternative option.

1. I use my cellphone to take pictures or video of the act. We all know how completely grainy surveillance footage can be. So I reach into my pocket, ensure the flash is off and begin taking pictures or filming. Heck, I may even live-stream to ensure the footage gets out.
2. I get on my phone and proceed to quietly call the police from my hiding place.
3. I remain quiet, remain hidden, and don’t say a word until the robber is gone.

Either route I take, I ensure to stay quiet and stay hidden from both the person robbing and the attendant.

Sure I can be a vigilante, but I have no idea if the robber has any friends close by. By acting, I could easily put myself and gas station employee in even more danger.
Calling the police is an option, but it’s worth mentioning that law enforcement won’t instantly show up. A LOT can happen in the 5 or 10 minutes it takes them to respond.

The absolute last thing I want to do is get myself or the employee shot. Above all else, there’s a significant chance that the robber just wants money. It’s insured, give it to him. Let him take as much as he wants and stay the hell out his way.

Once he leaves I comfort the employee, telling him to call the cops or telling him that I’ve got them on the line already. Once police show up, we both give a description that’s probably far better than the grainy surveillance footage. If I took pictures or video, I show them as well as will likely have to send them a copy as well.
Let me tell you, it’s far easier to give a description if you’re alive.

The entire time the ordeal is going on, I don’t present my weapon. The robber has a gun to more likely elicit fear than cause harm. While I could justifiably shoot the robber, I have to worry about missing and hitting the employee, my bullet going through the robber and hitting the employee, scaring the robber and he jumps and shoots the employee, my gun malfunctioning and now I’m left exposed, and a slew of other things. It’s not at all worth attempting to step in. Staying quiet and staying hidden increases the likelihood of both you and the employee getting home safe.