The wall in Lepage and Brassard's Polygraph is used in several ways. It separates Lucie's apartment from Francois'. (It is through this "wall" that David hears Francois' wracking sobs.)
The wall is also used as a means of separating the past from the present. We learn that David knew Francois many years before. David was investigating a homicide involving Francois' murdered friend Marie. For a time Francois is a prime suspect, and it is David that administers the polygraph test to the actor, though Francois does not recognize David six years later. However, Francois has not been able to leave the torment of that experience behind him.
The stage direction for this scene tells us that David slides over a wall. We might imagine that the wall represents a division in time, the past from the present:
David enters the restaurant over the wall, sliding down with his back to it, his arms and his suit jacket spread like a giant, ominous spider. David lands smoothly in the empty seat across the table from Francois.
The fact that David delivers the same questions in this scene as were asked at during the polygraph test allows us to recognize that François' memory will not allow him any peace.
The wall is also used at the Metro station at the play's end. Francois arrives at the station. The stage direction tells the reader that only the waiter's silhouette can be seen, "pacing impatiently." This image is project behind the wall. Lucie appears above the wall, acting out the "To be or not to be" speech Hamlet makes (in Shakespeare's play, Hamlet), as the young Dane considers whether to live on or take his own life. At the end of the speech, the lights on Lucie fade and the audience's attention is redirected toward Francois.
We hear the sound of the train coming...without hesitation, [Francois] throws himself in front of the arriving train. As his silhouette dives out of sight, the brick wall suddenly falls and a piercing light shines through as Francois' naked body comes hurtling through the falling bricks to land on a hospital trolley.
The wall has once again acted as a barrier, but this time between life and death. As Francois dies, the wall falls and bricks fly: the wall is broken, as is Francois' body...now very much like the body seen at the play's beginning that represented the murder victim whose death has (it would seem) caused Francois' suicide.