Overcoming what I've learned and practiced for nearly nine years is a daunting task. It is one that needs to be undertaken nonetheless. How does one know they're acting? To begin, restrict the scope of acting definition as just action, physical & mental.
Now that's not new, it's what Stanislavsky defined it to be. But to get here, is the challenge. In daily life we act sequentially in logical form performing day to day tasks. Every action prior to the one current, supports the current like the next action to come. If someone is making a cup of tea... A tea kettle is drawn out, checked for cleanliness and only then is water poured into it. It's boiled for a bit and then tea leaves are added, and waited upon to boil to release its essence then at a point milk is added and again boiled, all while simultaneously ensuring it doesn't spill over.
Now as an actor what if, you were to mime it! You'd probably go and repeat action after action and memorise it. Strain your muscles with the weight of the utensils and activity in sequence. Learning each action to do so and memorise it. This is acting in general. Everything despised by Stanislavsky as stock in trade and non-original.
Every time you repeat the action, your body memorises the muscular tension. Zero work inside, all the work outside. How does one proceed solving the problem of internal vs external acting? Physical vs Mental? The answer is through the sub-conscious mind and I believe may be called 'Ya-yesm'.
Have you ever tried soothing a tiny itch? Do you scratch it? As actors we know our best actions are the one's when they arrive from a space in which our mind makes us believe we need to act. You may wish to compare it to oneself forcing through the notion: acting is believing! This difference, is what helps one realise what is 'Ya-yesm'.
'Ya-yesm' requires you to not believe in your actions, instead it requires you to coax your sub-conscious mind into causing your mind to believe. Now how do I show this to you? What if I did? What if I didn't need to? What if you did? What if?