I would never take a guest on a restaurant shop without them knowing and being willing to assist. EVEN WHEN THEY KNOW they can screw up the shop unintentionally.
Recent examples with my long time restaurant shop partner:
Requirement: 2 beverages (no water)
Guest: "Just water with lemon for me."
Me: "You surprise me. I thought you wanted unsweet iced tea with lemon that you like so much."
Guest (coming out of fog): "You're right. Let me change that to unsweet tea."
Requirement: 2 different entrees
Guest: "That sounds good to me, I'll have that too."
Me: (Solid kick under the table) "I thought you were going to get the XYZ so we could both try both. (To server) Is it too late to change my order to the XYZ?"
Shop requirement: Server is to suggest appetizer
Guest: "I'll have an unsweet iced tea. And can we start with the ABC appetizer?"
Me: Sigh. There is nothing I can do to completely salvage my guest's blurt
During the shop itself I record timings because to the second timings are required on most shops. My guest is accustomed to my saying towards my microphone things such as "1:15:36 server check on appetizer. Beverage refill offered."
I do not have eyes in the back of my head and my helpful guest can tell me when a table behind me has been vacated so that I can time how long it takes for it to be addressed for cleaning and when it is reset and ready for incoming guests.
My guest is also useful to help 'spot the manager', help read name tags, confirm or dispute my estimates of age and height.
Not to be dismissed are my guest's comments about their food and swapping tastes. Recently my guest commented that their tomato tasted like fish. Huh? A small taste confirmed that indeed the tomato tasted like fish.
A guest can be an enormous help on a shop if they know what you are doing.