It’s the unpredictable. That’s what Brando had. The ability to surprise you with whatever he would do. From moment to moment you never knew. But it always made sense.
The Art of Concentration means giving your full attention to what you are doing. Concentrating on the other actor lets you experience the moment without thinking about your emotions. This is the creative source of acting. Your audience enjoys and believes you only to the extent that you are emotionally connected to the other actor. When you feel an emotion, you are interesting, but when you pretend or act an emotion, you lose credibility.
To concentrate, fix your attention on the other actor. The extent of your concentration ...