Like if I meet a new group or even a new dating prospect, I'll be myself as much as I can and then there will usually come a point where I feel like I've been "found out". There's usually a point where I spot them looking at me strange or pondering to themselves "what's up with this guy. why so quiet? he looks like he feels a bit uncomfortable?" If I feel that these things are going through their head, I feel almost obligated to say something like "I can be pretty reserved most of the time, so if it seems like I'm feeling out of place, I'm probably just thinking or observing what's going on - I warm up the more I get to know people". Something along those lines.
The people who respond to that with something encouraging or at least make a brief attempt to understand it/acknowledge it, I think I probably categorize them as being "someone with an open mind who I don't have to be so anxious around", but if it's clear that they think it's strange that I said that, then I'm usually coming to a conclusion that it would be difficult for us to be friends.
I've been told by many people that I should not say these things. Just leave it alone. It just feels normal to me - like I'm clearing the air of the awkwardness and letting them know "hey, don't think too much if I look uncomfortable - that's just me and I'll be fine". It feels honest and open.