Pupils look large hold of. Basically a flight reaction.
Your body has adjusted its gastro-intestinal tract for subsequent muscular activity. Digestion stops, the many sphincters (valves) in your digestive and elimination system go into spasm. Your body wants as much blood and vital nutriment as possible to be put at the disposal of the muscles. In extreme cases there may be involuntary emptying of the lower bowel and stomach (by vomiting); you may feel sick and be unable to face food.
A similar mechanism to above. Other muscles are also put on alert at times. Sometimes you can see them complaining (eyelids and calf muscles twitch spontaneously). Writing deteriorates – is almost unreadable at times.
The body wants to keep alert for the emergency it expects.
Pupils are dilated by adrenalin so that vision is as ‘wide’ as possible.
Jumpiness, overreaction to sounds -telephone, door rattle, and so on
The whole hearing system is primed to catch the first sound of an attack on you.