In this post headlined "Why Your Bartender Hates You" (provocative, eh?), Brown explains how things can get off on the wrong foot:
The incivility can be from both sides [of the bar], so I do acknowledge that sometimes customers are the victims as well as the perpetrators (and sometimes it's just one long chain). However, it's not uncommon for customers, bar side, to begin their interaction without even the basest of pleasantries. They shout "gin and tonic," an unnecessarily hostile beginning like a cannon ball fired from close range. This happens even in a quiet or slow bar.
While this method is direct, it's also a more appropriate first meeting for a vending machine. Why not, "Hi, can I get a gin & tonic?" or even, "Please." (Exasperated "pleases" don't count, either.)
. . . I was at an event recently making specialty cocktails and John McLaughlin, of the McLaughlin Group, asked for tonic water and lemon. When I tried delicately and politely to direct him to the bar that had those ingredients, which I did not, he threw up his hands in astonishment and made a gruff remark before departing the station. I did keep hoping he would say, "from zero to ten, ten being metaphysical certainty, do you have tonic water?" Of course my answer still would have been no.