Stanislavski's books are considered old hat these days. One can wrap his mind around a towering passion and still be an exceedingly bad actor. Some of the precise exercises of Meisner seem to point in the right direction. An actor must believe in the reality of his fictional situation and his fictional feelings and have a set of internal tunings that he can rely upon to be "the character" he is portraying. One must use oneself to make the character real, so always the character reveals something about the actor. But this is not important. The important part is achieving the magic.
Having been on numerous low-budget indie film sets lately, I am sometimes amazed when I see the resulting film and discover which cast members have given the most outstanding performances. Often the people who seem less "in character" or to be "too quiet" during a take turn out to be incredibly effective as they are recorded by the camera. Big-motioned energetic actors may come off as "too stagey" for the film reality. And yet, some actors that seem too intense or stagey on set can be somehow transformed, when the camera so wishes, into powerhouses of visual interest. Thus the mystery-- just craft, or something unpredictable, ineffable?
Does the camera love you or not? I think an actor is very lucky when it does, but I also think this is not true for everyone. Camera and actor must also be in sync, and sometimes the lens misses the performance and thus loves you not. An actor with strong and invisible camera awareness can get the greater love.
Some say that the camera misses nothing, and that it can never be fooled. I don't believe that. The camera sees only what it is given during a take. A camera is unblinking yes, but multiple takes and editing can cheat to the actor's favor. Often times the actors that can very consistently give the least to the camera are the most interesting. Woody Allen asks his actors for "less." I agree this should help because it is easier that way to achieve consistency, and in film reality, consistency is king. Consistency of character, when absolute, is in some sense fooling the camera by with-holding any untrue element from the character. As George Burns said: "Acting is all about honesty; so if you can fake that you've got it made." The camera must see authentic human action, not "acting." And yet, at the same time the actor is like any other magician who distracts the audience so that they see the magic and not the "trick." Great actors, I think, have found a way to make peace with that, to embrace the Yin and the Yang and deliver the good stuff.
I think the actor's path is of two sorts. If the camera loves you right off, you have time to develop craft without doing much at first. Lucky you. You are a mask actor, and your mask is beautiful. If you are a character actor and the camera loves you not, you must invoke every stratagem to get from the camera at least a grudging consideration. This is honest work, and it is by working that you gain the camera's respect. Eventually the camera may recognize you as a useful stereotype, and that is not a bad thing. It can take years for an actor to master that-- evoking the needed response from a minor but necessary bit that sells everything else.
Despite all the chatter and analysis, the true mystery remains. The alchemistry of the captured moment is all that matters, and that depends on much more than just the actor. Trumping the question of whether the actor gave a great performance or not, is the question of whether the desired performance exists on film, and why or why not? What the director wants, asks for, and settles for plays a crucial role. Editing also determines what parts of a performance make the scene, how the film is paced, and whether the bits adhere into a film containing effective moments.
The true answer, then, about the mystery of film acting is that investigation reveals more of a context, a gestalt, than a straight-forward explanation. Achieving the magic requires intense work and coordination from everyone. Thus the words: let's try that just one more time.