Most theoreticians consider the 20th century's theatre the theatre of directors. It is in this century that performance theories became especially important, and the place of the main elements of theatrical creativities (play script, actor, director, design, costumes were studied and evaluated from different points of view. One of the most interesting theatre debates in this century deals with the place of actor in theatre; having a variety of schools and methods, the subject of the actor's work has developed to a point that is considered a classical debate. One of the first theatre practitioners, who had theories about acting and actors, was Denis Diderot in 18th century. His books, especially Paradox surle Comedien (The Paradox of Actor), were collections of his theories about imitation and acting, which were used until the end of 19th century and it is under discussion even today. The 19th century was a period of forming and developing theatre in its modern sense, and the re-evaluation of the importance of actor's role was essentially rooted in theoretical approaches to theatre in that century. Some of the influential directors who had affected the way of acting in theatre were Andre Antoine, Firman Gemier and Logne-Poe in France, and also Constantin Stanislavski, Nemirovitch Danchenko and Vesvolod Meyerhold in Russia. Among them, Stanislavski was the most important one, whose books (An Actor Prepares, Building a Character and Creating a Role) about his actor training system, made a real revolution in theatre and acting method. Between the two world wars, theatre theoreticians, like Antonin Artaud and Bertolt Brecht, proposed new ways in acting, which were very different from realistic acting. For example, in Stanislavskis system, the actors must imagine a fourth wall (an invisible wall) between themselves and the audience, while in Brechtian theatre "alienation" (verfremdung), meaning putting away the forth wall, imitation and illusion of reality, was introduced. In the beginning of 20th century, many directors paid attention to some old acting styles like "comedia del arte" and also Oriental acting methods. One of them was Meyerhold, who using some of these obsolete styles in his works innovated a new acting style called "Biomechanical Constructivism". Edward Gordon Craig was another director and theoretician who in referral to oriental actors, wished to substitute Sur-marionnettes instead of actors in order to free theatre from literature, music and painting. This article is an attempt to reflect such approaches of those directors while studying the conditions of actors at the end of 19th and beginning of 20th century, and also the effect of the 19th century directors' theories on the thought of the directors of the 20th century, especially where it relates to the work of actor. As a result, we can not name one specific method of acting in contemporary theatre. We have many styles and methods, and to obtain a new definition of actors work, we would have to put together and mix many different styles from different directors.