I think a lot of movie cast directors would disagree. If it was that easy, then more Hollywood actors would just be scooped off the street as nobodies and trained to greatness. Again, not saying you can't teach it... but some people are clearly born with Xfactor you just cant teach. You can only train to move your body to move so much. You can only augment with make up and costume so much (assuming you have a budget). I saw some untrained zombies give a performance that i have never seen in any zombie movie from professional actors.