NBA refs talk to coaches and smile, talk to players and smile. So do NCAA refs, so do HS refs. Not all the time, not every ref, but enough for me to notice it watching games. And they all look very comfortable and professional.
I think that it is important that you maintain a good body posture and professional demeanor, but you don't have to be a stone face. I think it is more about how you carry the rest of your body that people notice and judge you for. Nothing wrong with enjoying yourself, but just beware of body language that sends the wrong message.
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