>>94232 (OP)
It would probably be even scarier.
Anyhow, here is the recommended list of phrases to keep in mind when interacting with a cashier:
>hello
>yes
>no, thank you (, I have a bag/backpack)
>cash
>I have a card
>this is a robbery
>put the money in the bag
>hands where I can see them
>yes (, I would like a coupon/sticker)
>thank you
>y-you too
>goodbye
These cover the majority of everyday scenarios. If a situation that cannot be covered by either of these is encountered, the safest bet is to pretend you've gotten an urgent phone call and flee.