If performing a show in an extremely hot costume or with a medical condition, make sure that you have the following: water, fans or AC, a light snack, a drink with electrolytes, and a crew member on standby. The reason for this is if you feel that you are to pass out, a crew member will alert the director. Water and fans will help keep you somewhat cool and if you have plenty of time between scenes, open up your costume to let air flow in. In the event you do pass out, when you wake up, a drink with electrolytes and a light snack will help remove any remaining symptoms of dizziness.
Hope this helps! Break a leg out there!
Theater Bee 