Bill Murray -- 1 Tequila, 2 Tequila, 3 Tequila, 4 ... (VIDEO)

Bill Murray did his best impression of a bartender Friday night in NYC ... and tequila was a staple on the menu.

Murray was tending at 21 Greenpoint in Brooklyn -- a bar opened up by his son -- and George Clooney's Casamigos Tequila was flowing like water.

We're told Bill stayed for a few hours ... Clooney couldn't make it but picked up the tequila tab.