Relax. Great actors spend their entire careers learning to relax the muscles in their face and body at will. Tension is very obvious when you are on stage. Your voice will sound thin and wavering, and your movements will be jerky and unattractive. To avoid these stiff and nervous displays when you're acting, it is essential to remain as relaxed as possible. Even a scene involving high drama calls for measured and calm concentration from the actor. So, act dramatic, but be calm inside, and don't work yourself up.