Final answer:
The opening of the curtain by Don, Cosmo, and R.F. during Lina's live singing likely exposes a comedic deceit, such as lip-syncing, in the context of the movie or play involving The Dueling Cavalier.
Step-by-step explanation:
The question refers to a fictional event from a movie or a play where The Dueling Cavalier is presumably a show or a film within the story.
When Lina starts singing, Don, Cosmo, and R.F. reveal a trick by opening the curtain behind her, presumably exposing some form of deceit like Lina lip-syncing or the real singer being behind the curtain. This pivotal moment is meant to be comedic and exposing, causing embarrassment to Lina and amusement to the audience both within the story and watching the movie or play.
The given passages, however, do not indicate a direct answer to the question and are unrelated to the specific event described.