The act you put on in public or around friends and strangers. Anyone that isn't family or that doesn't know how you really are thinks you act one way and only that one way. Likewise, your family may think you act a different way and only that one way. Often times, the friends and family will not know of both personalities or "acts". These "acts" can be made up of multiple characteristics, usually opposites. There are countless different "acts" to fit everyone that has one.
Frank: "You can trust me, Steve, I won't let you down."
Steve: " I don't know about this, Frank. You have been known to mess up and mess others up."
Frank: " Yeah well that's just my social act. I really am responsible."