I took one acting class as an undergraduate and was made keenly aware of the fact that I can't act.
However, I did learn one very important theater term that has helped me in my writing endeavors, and that is "blocking."
In essence, blocking refers to how actors move and are positioned on a stage. Where are they standing in a room? What are they doing in that space?
Blocking also refers to the posture and gestures used by an actor. For example, you can display the character trait of laziness through a slouch, or nervousness through hands that are never still and fingers twisting together. Proper use of body language sets the mood of a scene.
I believe blocking is a useful concept for writers because it's easy to get bogged down in lines of dialogue without having concrete references to what characters are actually doing in the space they occupy while conversing with other characters.