Speaking to guests:
When you work as a barback you are supposed to greet the customer, but your main focus should be helping the bartender/waiters and make sure they always have what they need. If a customer starts talking to you, then be polite, speak clearly and keep eye contact. if you cannot answer their question, get a bartender, waiter or manager and they will take it from there.
If a customer asks you to take an order, you politely tell them that you cannot, but you will get a bartender to service them.
Half empty glasses?
If you see a guest that has a half-empty glass or less, and they have not touched it in a while, you can politely ask if you can remove it.
Placing people at tables?
If a waiter asks you to seat a customer at a table then they will show you where to go. If a customer asks you to give them a table, then politely tell them that you will get a waiter to do this as soon as possible.
Tap on the shoulder when moving behind co-workers
A tap on the shoulder is a sign that you're behind the person you tapping. Whenever you walk behind a bartender or someone else that doesn't see you, give them a tap on the shoulder to let them know you are behind them.
Too intoxicated customer?
If you believe the customer is too intoxicated, you should inform a security guard if they are in the establishment. Otherwise, you should either inform the manager or the bartenders so they know to stop serving the customer. It is not your job to tell them to stop drinking or to throw them out.
If a customer is aggressive then get the manager or the security. It is not your job to calm him down.
If a glass, plate or something else gets broken, the first thing you want to make sure of is that the guest does not touch it, and let them know that you will take care of it. Get a dustpan and brush to sweep up the broken glass and clean up the food/liquid.