What is Back Acting? Well, in film/television it is acting when your back is facing the camera. It is an over the shoulder shot, and it's over your shoulder.
We really don't put much attention to this style of acting. But it is very important to know that it is important. What is your back saying, what are you thinking, are you in the moment? Or are you making funny faces to your scene partner while he/she is trying to cry on camera. Or are you just delivering your lines because now it doesn't matter how you say them since you are "Back Acting".
Back Acting on stage is, yet the same premise as the one we just spoke of, but it goes one step further. You are saying your lines, you start to walk around the stage, and somehow you finish delivering the lines to the wall. You have forgotten about the audience.
In both scenarios it's important to be aware of what you are doing. So you can make the decision a right decision to make you a better actor. Either you'll continue making faces or you'll decide to deliver your lines convincingly and help your scene partner get to that emotional peak.
On stage, it's good to be aware you are Back Acting and when. Are you Back Acting when a scene is too emotional for you, are you feeling uncomfortable, what is going on in that moment that is making you turn away from the audience? Know that when you want to turn away is exactly the moment you should look directly out to them.