Before reaching the point of being prepared to undertake a role in a theatrical production, a performer must undergo extensive training. This training is designed to provide him or her with the ability to move onstage with ease and authority, speak clearly and project so as to be heard throughout the theater, and create and project the inner life of the character. In addition, the actor or actress must interact with other performers, convey both the playwright's and the author's intentions, and wear costumes with poise and assurance. Many serious attempts have been made to define and teach the art of acting, but the most influential modern system was created by Constantin Stanislavski. Stanislavski focused on making the character being performed appear believable. He achieved this by using techniques centering on relaxation, concentration and observation, concrete activities, inner truth, emotional recall, onstage action, and a role's through line. In addition, he placed great emphasis on ensemble playing. Later in his career, he explored the effects of psychophysical actions. Special skills are necessary for certain types of theater, such as musical theater, pantomime, stylized theater, and avant-garde theater. However, no matter what type of theater is being done, performers must develop the sensitivity and insight to penetrate the secrets of the human soul, as well as the skills necessary to communicate those insights to the audience. They must learn to integrate the inner feelings of the character with the outer expression necessary to communicate to an audience.