Communication is vital to all living entities; would flowers be fertilized without signaling to insects?  For people, lying, withholding and denial create poor health, while speaking the truth and listening for it allow us to access great personal power and to enjoy dynamic, fulfilling relationships.  Authenticity happens when we express what is true for us in words, in our actions, and by the way we move.  Authentic communication also means listening in a way that invites others to be authentic: hearing others accurately, and also noticing the feelings and desires that inspire their words.  One way we can be authentic is to notice and acknowledge our own inauthenticity: the roles we play, the things we want others to believe about us.
The Basic Shift:  Telling the Unarguable Truth
The unarguable truth is a truth that does not produce argument.  It is an attitude as much as a technique.  A good starting place is describing your personal experience as literally as possible.  Start with physical sensations ("my palms are sweaty"), then go to core emotions ("I am feeling scared/sad/angry/joyful/sexy") and finally acknowledge thoughts ("I am thinking that you don't like me.").