In Lepage and Brassard's Polygraph, the "echo" between the death Lucie witnesses and Francois' suicide first serves as the presentation of foreshadowing and its resolution at the story's end.
Lucie witnesses the suicide of an unidentified man who throws himself in front of the Metro train. It is during this tragic event that she meets David, the official who comes to investigate the scene of the man's death.
Francois is introduced into the present day as the two neighbors discuss Francois' boyfriend, Alain. Lucie wants to know if Alain is mad at her for some reason, but Francois puts her mind at ease:
No problem. It's me he's mad at.
There are many intriguing details included in the play. Francois is described as predatory; we never hear of Alain again. One wonders if the pair broke up, or if Alain was the man who died in front of the train. Francois cries and laments, his wailing heard by David through Lucie's apartment wall—the apartment that adjoins Francois'. Is it guilt that haunts him? Or the thought that people believe he is guilty?
Lucie, to her horror, finds out that she is playing the part of Francois' murdered friend in her film. She is also dating the man who administered Francois' polygraph.
On a date with Lucie at Francois' restaurant, David recalls having given Francois a polygraph (lie detector) test six years before while investigating the rape and murder of a young woman who Francois knew well. For a time, Francois was the chief suspect in the case. And although he was ruled out by the authorities of any wrongdoing, he was never told of the test's results, which seems to have damaged him psychologically—if we can assume his innocence. (The audience is never certain if he was guilty or innocent.)
When Lucie and David speak to Francois about the suicide, David comments on the large number of suicides by train. (This is the foreshadowing.)
It's the cheapest way to kill yourself.
David's question of "What?" is never answered, but we might infer by the end that this is something that Francois knew about—having either considered suicide or knowing someone else who had...or was he simply making small talk?
We learn of Francois' breakdown while he was being questioned during the inquest of Marie-Claude Legare. It comes to the audience's attention that strangely enough, it was David that had administered the polygraph test to Francois in Quebec City. The murder, the suspicion or both devastated Francois:
(a complete emotional breakdown)
...I didn't kill her!! It wasn't me! It wasn't me! You want to drive me mad, that's it!! You are driving me mad...
Toward the end of the play, Lucie asks Francois if he killed Marie. Francois is not sure anymore, and something David had mentioned in a presentation about polygraphs makes the audience wonder if Francois was driven to a fragile mental state because of that experience:
...the psychological response we trigger is so violent as to effect a lasting disorder in the mind of a totally innocent suspect...
There is nothing that shows any connection between the first suicide victim and Francois. We can infer that if nothing else, both shared a feeling of desolation that each could no longer live with. But the echoes can be found in Lucie's role reenacting Marie's death, David's reappearance in Francois' life and, ultimately, in Francois' death that is so similar to the one Lucie witnessed.
Lucie and Francois are connected at several levels of parallel that "echo" from one to the other character. (1) They are neighbors. (2) Lucie is cast in the movie as the woman whom Francois was suspected of having murdered years earlier. (3) Lucie is dating the man who the pathologist at the woman's autopsy.
Since their lives echo each others', then it is logical their experiences of death will echo each other. In this case though, the echo is on Lucie's side all the way through while the reality is on Francois' side: his polygraph test, his burden of suspicion, his death.