People skills are not easy and I’m not sure they can be taught. I certainly know someone who make you feel uneasy just by sitting next to you. (luckily he only shoots landscapes)
I go for the cheeky approach. It’s much easier if you have a chat beforehand but if that’s not possible, I have a handful of “old classics” that get most people smiling. You may not want them smiling but it’s better than stiff.