Every single day is a battle for me not to give someone back the litter they threw on the ground. But when you try to combat people's anarchic social behavior with snarkiness, you basically just up the ante, and who knows what kind of reaction that will provoke?
So the Posts’ first rule is to pick your battles when you call someone out. If they are just being rude in general and you happen to have a front row seat to the chaos, maybe don't get involved. But if the rudeness affects you personally — they’re in your seat, cut you in line, etc. — it’s open season.
But if you do intervene, they say that you shouldn't be rude back. Instead just point out the error: "I think you're sitting in my seat" or "Excuse me, I think I was in line before you." And you can also just ask kindly: "I know it's tough for little kids on long trips, but can you ask your son to stop kicking the back of my seat?"
*I'm talking about general, low-grade rudeness here. If you see someone being bullied, or you're witnessing hate speech or harassment that's a different thing entirely.