So, with the attorneys I have worked for/with, it is common practice for an attorney, when listening to someone explain/tell them about an issue, to put their hand to their face…. generally thumb under jaw, first/pointer finger over the mouth (what we are seeing BM do with his hand while listening to AM testify). This is to remind them NOT to visibly react to any information they hear. It is a practiced mental/physical cue they practice so they can maintain their composure to whatever another person is saying.
I am not an attorney, but I have witnessed this behavior in a multitude of situations.
moo