In answer, so much depends on the situation. Clearly, standing on a platform in front to a large audience is different from getting on a train and smiling at fellow passengers.
I almost needn’t say, first impressions are incredibly important. Every textbook echoes that line. For all our advancement and sophistication, judging people in the first couple of minutes we meet them is imbedded. Maybe it’s some ancient human defence mechanism that discriminates between friend and foe. We can be ready to embrace or flee at the drop of a hat.
There’s the core of an answer. First off, in most situations, the impression to give is that of a potential friend. If the people in question are openly threatening, then precisely the opposite comes into play. Call it “caveman” but these human responses are intrinsic.
John Vincent 
These human responses are intrinsic indeed! The fight or flight syndrome. 🤔