The scene that Athena and her partner are working on is from William Shakespeare's "Hamlet," specifically the Act 2 conversation between Hamlet and his friends Rosencrantz and Guildenstern. The film's selection of this scene is very much intentional, as Sam enters the rehearsal room just as Athena recites the famous line "O God, I could be bounded in a nutshell and count myself a king of infinite space, were it not that I have bad dreams."
It's a pointed reference, especially in hindsight; Henry's fantasy encompasses the entire world and the lives of everyone we see, but ultimately only exists in his dying, haunted mind. There are a number of "Hamlet" references throughout the film: The bystanders who become players in Henry's dream world resemble the company of actors Hamlet utilizes to "catch the conscience of the king" with their play-within-a-play. Henry's last name, Letham, is an anagram of Hamlet, and like Shakespeare's melancholy Dane, he is a mopey dreamboat poised between action and inaction, life and death.
Hamlet's "To be or not to be" monologue is perhaps the most famous consideration of suicide in the English language, as he waffles (to pretends to, depending on your interpretation) between duty to his dead father and the desire to end it all. The film makes that predicament literal, placing Henry and his infinite kingdom in a liminal space where he could live or die by the next moment.
