It's hard to know if I'm doing it right...! Sometimes the customer will disappear when I give him a brew, sometimes they won't. The lack of feedback makes me think it's random as to what will work and what won't.
The visuals are nice, and the voice lines are pretty good (I got a chuckle hearing Terry hum the Funeral March when a customer left the store), but I think it might be better if they only played every once in a while; or at least, didn't overlap with each other.