It's like learning to fly an airplane: you master the various instruments in the cockpit which are comparable to the elements of a scene. You then move on to a scene and the emotional content which is required. These skills are the foundation of a technique. I do not spend time discussing theories. You execute the instructions so that you begin to master technique. This includes making mistakes. Mistakes are wonderful learning opportunities.

Acting is not a spectator sport. You must be in the process.
Of course, observing others is a form of secondary learning if you know what to look for.