What does the audience know that John doesn’t?
The audience knows that Sarah is not going to be at the party John is preparing for her, as she is on her way to a different party. This creates a situation where John's excitement and anticipation will lead to disappointment because his surprise will not have the intended effect.
How does the dramatic irony affect the scene?
The dramatic irony creates tension and anticipation for the audience, as they are aware of Sarah's absence while John is blissfully unaware. This knowledge amplifies the emotional stakes, making the audience feel sympathy for John as he prepares for a celebration that will not happen as he expects. It also sets up an ironic contrast between John's hopeful intentions and the reality of the situation, which can elicit a mix of humor and sadness from the audience.