Its usually easier when in a group of 3, like in lunchbreaks etc, then you can just watch quietly in the background and just join in when the conversation is already rolling. If I can avoid awkward situations where im alone with someone else, i do. Those are difficult for me because there's no 'buffer'.
When i have nothing to say, like Recluse' Ok Thanks, if im in an ok mood, i usually raise my eyebrows and make my eyes / mouth smile even if im saying absolutely nothing of value, to signify that im open and approachable.
If i am not disturbed by something or don't dislike a person, i am very friendly in public and approachable. i think i developed this habit to contrast against a couple of family members of mine who were mean in public and it bothered me.