Luigi Pirandello's "Six Characters in Search of an Author" .

Eric Bentley credits Pirandello as being the first playwright to actually use the stage as a stage. In his most renowned masterpiece "Six Characters in Search of an Author" Pirandello uses stage in the most unique way. The play is an absurdist metatheatrical play about the relationship between authors, their characters, and theatre practitioners. It premiered at the Teatro Valle in Rome to a mixed reception, with shouts from the audience of "Manicomio!" ("Madhouse!"). I wonder how many in the audience realized that the "madhouse" they had witnessed was a turning point in the history of drama. 

When the lights come up on Luigi Pirandello’s Six Characters in Search of an Author (1921), the first thing the audience sees is a bare stage, with no scenery and only a few folding tables and chairs scattered about. The curtain is up, the stage is empty of props and background, and the lights illuminate the bare wall at the back of the stage. A stage-hand is starting to build a set, but the stage manager interrupts him to say that it is time for rehearsal. The producer and a company of actors arrive and begin reading out stage directions; the actors complain about the script, but the producer (who also serves as director) explains that he “can’t get hold of good French plays any more so that now we’re reduced to putting on plays by Pirandello.”

The play they are rehearsing is called "The Rules of the Game". It is an actual play by Pirandello. The technique he uses here is called "play within a play", which is an ancient concept and a frequently used technique in theatre. But in this play Pirandello took the  metatheatrical games to a new level. 

Suddenly, this ordinary rehearsal is interrupted by the sudden entrance of six characters. These characters are simply identified as Father, Mother, Stepdaughter, Son, Boy, and Girl. They are six unfinished characters who have stories to tell, stories of their lives, but it is not fully written. They were abandoned by their author so they are searching for a new one in hopes of completing their story. As Pirandello said in his 1925 introduction to the play: "Every creature of fantasy and art, in order to exist, must have his drama, that is, a drama in which he may be a character and for which he is a character. This drama is the character's raison d'etre, his vital function, necessary for his existence."  

The play than proceeds with the characters telling their stories, some scenes of incidents in their lives, which are accompanied by comments, quarrels, dialogues, interactions among the characters, between the characters and the actors, the director and the characters etc. It creates a theatrical Hall of Mirror effect. The actors who will acting the parts of this six characters become the audience of their life story. For them the stage becomes more real than the world and the line between reality and acting, imagination and fiction becomes blurred.