You're confused.
Case in point: in Lord of the Rings, Peter Jackson comes to me and says ‘I would like you to be Gandalf the Wizard,’ and I said ‘You are aware that I am not really a wizard?’ and Peter Jackson said ‘I would like you to use your acting skills to portray a wizard for the duration of the show.’
So i said ‘Okay’ and then I said to myself ‘Mmm.. How do I do that?’ And this is what I did: I imagined that I was a wizard, and then I pretended, and acted, in that way on the stage.
How did I know what to say? The words were written down for me in a script. How did I know where to stand? People told me where to stand.