It’s a really terrifying situation. It happened to my brother when he was working at a newsagent’s, and someone pointed a gun at him. He stayed calm, but when he told me about it, I was absolutely terrified. The town where he used to live had become really dangerous!
I believe that showing calm is the most advisable thing to do in moments like these, even if inside we are terrified; it is better to do everything slowly or even remain motionless if we are asked to.
That’s definitely the best option.