Open in terms of body language basically means facing the other person directly with nothing in between you and them. The front of our body (ventral side) is the most vulnerable side so when we feel threatened we cover it with objects or limbs or we turn it so that less is directly showing even whether we sit behind or beside a desk. If you completely expose your ventral side to the person you are talking to you are seen as having open body language. People will see you as 'open' and relaxed and will in turn be more open and relaxed around you.
It follows that an open face would be completely exposed to people you're talking to. That means squarely facing people who you want to see you as open, smiling, looking at their face and not the floor/wall/your knee.
The exposure of our ventral side is the big one though, it just shows that you are comfortable with the other person in turn making them comfortable. I've read that even exposing the ventral side of the arms and wrists (inner side) will put people at ease but haven't had the chance to try it. I've tried the others and they do work.
And slightly raised/arched eybrows instead of frowning too. Not having 'shifty' eyes, eyes that are constantly moving as though you were watching for predators.
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read all the pamphlets and watch the tapes!
get all confused and then mix up the dates.