One time I impatiently but under my breath said something shitty ( like, can we move this circus along here?) about a very annoying but harmless person in a store. The cashier looked at me, gave me a little smile, and said, “Don’t be like that.” And I realized that I could be better, more patient.
Now, I don’t think it’s a great idea in every situation, but I have tried it with people who believe they are “nice people” and they realize they are not proud of themselves and that they’re being noticed being ugly.