Someone seems phoney and we condemn them- 'he's a real actor, it's all an act'. Someone is ill and they put a brave face on it and refuse to burden others with their woes in situations where burdening isn't useful for anyone- that person is a model citizen. It all depends on the act, your acting. What is this acting we do in everyday life? (I am reminded of a line in the movie My dinner with Andre; 'Grotowski gave up the theatre because he thought we were all acting so well all the time anyway...') What about the person who says- what you see is what you get- I don't hide anything? What they hide is their desire to hide things...Being upfront about everything is a kind of crudity, certainly it means you are reading your own signals before being sensitive to those of the situation. Of course you can be TOO sensitive...Politeness is supposed to be an act, so are good manners- but the best manners are simply being considerate- which you can observe in the poorest, and richest, of people. Of course you can be TOO considerate...Acting is following a template or model of behaviour for a situation rather than re-acting using habitual responses. You either act or re-act, you can't escape it. When you're acting you may get 'stuck in the role'- my character just wouldn't do that actors complain to a writer. Make your character do it! You're surrounded by friends and family and feel very relaxed- how do you act? I imagine you suppress half unconciously certain aspects of your life that won't quite fit in in this environment.
In the traditional world your act is given to you on a plate. Everything about a traditional society has survived for good reason- and it breaks down for good reason too. You can't go back, however to observe it you can see that the demeanor, words, clothes, rites of passage, manner of address are all programmed into the society rather than, as in a modern 'free' society being largely the responsibilty of the individual. Of course you have to 'act' by putting on a suit, being polite to your boss- but the act is often resented, if only you were a millionaire you wouldn't have to act at all! But an actor doesn't resent his costume, he just tweaks it so that it 'works' for him- or her, the actress Archie Punjabi insisted on always wearing boots when she played a certain role- it made her feel tougher, which enhanced the act. So those people who are always 'themselves' might just be using props that allow them to stay 'in character'. So become aware of props, costumes, mannerisms which make acting a certain way effortless.
As a teacher in front of a class of children you have to act- you cannot react. People who are 'cool' are those who have habitualised certain responses early on in life. It looks like they aren't acting which is why they sometimes make good actors in movies.
You can look at acting as deciding on a certain persona or stance you conciously adopt for which you face most situations in life. It's OK to go with the flow, but it stops you from drowning if you have a boat. Doesn't have to be that big either.