I was working at a rave on the weekend and consciously working on my posture and making eye contact with lots of people. But a few comments from co workers made me realise that my 'serious / concentration' work face is actually probably quite offputting. 'are you ok?' 'don't look like you're having such a good time (sarcasm)' etc. I'm not actually in an unhappy mood, enjoying the music and vibe if anything on this night, but I may as well have a 'serial killer' sign on my head, apparently. So this isn't really asking for advice but if you think you might come across as threatening then you might have a similar issue. I suppose all I can really do is try to turn my mouth into a semi smile every time I think about it just like I do with posture / straightening up.