Many theatre students think that their reactions need to be "big" to be seen from the audience. The truth is when you try to show emotions in a cliche way, you alienate the audience and provide them with an unrealistic performance.
Tip 1: Have an active mind. Try to put yourself into the mental state of the character you are playing and think the thoughts that he or she would be thinking.
Tip 2: Don't pre-plan every emotion. Each rehearsal and each performance is unique. Allow yourself to feel differently at different times and to react truthfully in every moment as you listen to your castmates deliver their lines.
Tip 3: Try to hide strong emotions. Normally when someone is angry or upset they try to hide that emotion. Use that same impulse on stage. It's much more believable to try to avoid crying when you are sad or to be angry but fighting to stay calm. This is how humans act in their daily lives and so we want to replicate that in our performances.