Shower Them with Kindness

You won't change them, but you can surprise them :)
When I was a waiter almost twenty years ago, I had many encounters with a wide variety of guests: nice, angry, happy, sad, appreciative, unkind, and some that felt downright evil.
This is nothing new. Every waiter has their own set of encounters and stories they could share with the world, and many would blow mine out of the water.
The thing is, you can pretty much tell right away how an interaction will go with new guests, and you realize quickly how you must adapt or you're in for an hour of hell, give or take.
I've never once done anything inappropriate to a guest's order, food, or drinks. I both thought it was the wrong thing to do, and I believed that if I did something, someone would eventually do the same to me.
Instead, I flipped the script and showered them with kindness. I went out of my way not only to attend to their requests but to check in more frequently, ask for their needs, refill their drinks before it was needed, and give them an A+ service they weren't expecting.
I did the same for all customers, but the unkind guests were so undeserving that the contrast stood out even more.
In the beginning, they're caught off guard. Suspicious, cynical, and closed off. But that changes quickly. As you're nice to them, and you smile, and you actually take care of them, they turn from cold to warm. They begin mimicking you: smiling back, asking how your day is going, and keeping a friendly conversation going.
Of course, this doesn't happen every single time. But such conversions were so frequent that it became a personal mantra the moment any difficult guest sat in my section. I would tell myself: shower them with kindness, treat them well, be nice no matter what. Most of the time it worked. Rarely it didn't, with those guests going out of their way to make your shift hell and your day miserable.
But another discovery I made was this: once converted, once I had befriended such a difficult guest, they didn't just turn friendly. They were fun, happy, and left me an amazing tip :)
And the next time they visited, we picked up our friendly conversation right from the start, with no thawing required whatsoever.