Acting is Observation and Reaction.
Everyone in this photo is watching someone else intently. No one in this photo seems to be worrying what s/he looks like, or what to do with his/her hands--everyone is paying attention to someone else. Young actors often wonder "what can I do to look like a really good actor on stage?" The answer is: make your scene partners look good. That means reacting to and responding to everything that they do on stage. If your scene partner wilts, or smiles, or looks at something with concern, you can respond to it if you are paying enough attention to observe the action/reaction in the first place.
In real life, when you want something--really, really, really, want something--from someone else, you pay very close attention to them to guage whether your efforts to get them to do what you want are working. If you pay this kind of attention to your scene partners, you'll be able to react and respond to everything they do on stage.
Let's say your scenie is trying to frighten you, but he isn't very frightening. It does not make your scene partner look good if you shrink back in horror and start cowering in terror when your scenie is just standing there, even if that's how the scene is "supposed" to go. But if you are watching your scenie closely enough, you'll catch the moment when his body language shifts and briefly becomes more menacing, you'll catch the quick moment when a real fierce look comes over his face, you'll pick up on that quick surge of energy when he makes his attempt to frighten. If you observe your scene partners carefully enough, you can react to the genuine moments when they are doing something on stage--in the EXACT MOMENT that those things happen. I say it all the time, but a scene exists in the air between the scene partners--how they affect each other is what makes for compelling theater. The more closely and carefully you pay attention to your scenies, the more you'll be able to respond to what they are actually doing the moment they do it.