Practice with others.
You can approach strangers in situations where making conversation is permitted, strangers you detect with the same type of strong looking faces, gestures and mannerisms. The guy selling hot dogs at the corner, the guy selling balloons at the park, the woman selling lottery tickets, the officer directing traffic.
Pick one or two people and follow through not just on a single day but recurrently, make conversation, get to know them so you detect what kind of ideas they are prone to defend and challenge their ideas trying diverse strategies.
This doesn't mean to get in arguments, it doesn't mean to contradict their ideas, challenging is as simple as asking them to explain, to elaborate, then you can go further if you feel this won't end in confrontation, but also don't be too afraid to be out of your comfort zone in conversations or push them (your subjects) a little outside of their comfort zone, after all, you need to practice for when you need to do the same with managers and they (your stranger subjects) don't have a say in your job. Doing so, you are getting more and more skills and more experience on how to deal with strong characters like managers.
Soon you will find out you now feel more comfortable exchanging info on important sensitive matters with mangers and such, because you have been developing confidence practicing those interpersonal skills you mentioned in your question, and you will learn more techniques along the way.
You'll also end up making new community friends around your public area.
Best of luck.