You can’t always plan it.  When someone lets their inner self shine it is not something that you as the photographer can control.  While there are a few tricks to coax it out it is essentially up to the model to let you see it and up to the photographer to be ready to catch up.  It is sometimes nothing more than accidental beauty, they are not posing or giving you a practiced facial expression, it is them.  Shine On.  Shining through.