On stage, if an actor experiences only internal feelings or only physical actions, then the performance is dead. The reasoning behind this goes back to the union of the psychological and physical. The two go hand-in-hand. If an actor attempts to portray a character by employing one aspect of the union without the other, then they are performing incompletely. Internal experiences and their physical expression are unbreakably united. Whether it is through a facial expression or the tapping of a foot, everything a human experiences psychologically is displayed through physical means. This is termed a psycho-physical union.
The correct physical action does not come automatically for every psychological response nor do they stimulate identical responses for every individual. Many times, actors need to experiment until they determine what best works for them and for the character they are trying to portray. The best way to experiment with this is through improvisation. The best improvisers are those who can intuitively act and behave onstage as though they are in a real situation.