My theory of writing and my theory of acting are more or less identical. Since I have never actually acted (not yet, anyway), let me state it in terms of writing. In short: write with meaning. Write something that is important to you. Whether you are being sarcastic, or earnest, write something that means something to you.
As far as acting goes, I have never heard it stated better than it was by Jack Lemmon, during his appearance on Inside the Actors Studio. Remembering advice he received in the start of his career from James Cagney, Lemmon stated the key to acting this way: Learn your lines, find your mark, plant your feet, and say your lines — but mean them.
That simple philosophy has always made since to me, and it has been a guiding principle to me as a writer. But to lots of great actors, it is a good deal more complicated than that. Enter the acting class, where thousands of would-be stars of stage and screen come every day in hopes of learning and honing their craft, and which also provides the setting for this very, very funny short film starring Joe Urla:
Was that a long way to go just to set up a video, or what? Jesus.