Come onstage, joke with each other very naturally all through the recitative.
When the Countess says ‘Scribi’, she points to the Count’s desk. Susanna gets the quill (should be a feather, and dipped in what looks like an inkwell) and paper, and comes back to the Countess who stays on that side of the room.
The Countess is suddenly aware of the orchestra, and right before the Countess says 'Canzonetta Sull'aria', she points to the pianist and comes downstage. Sits on the floor, as though she’s lounging on a meadow. Susanna eases down herself into the pit, where she proceeds to writes on top of the piano (let us pray for a real piano…). After she goes down into the pit, she says ‘Sull’aria.’
The Countess says everything here in a daydream. Susanna looks like she’s concentrating very hard. Looks up at the Countess to make sure she gets every word, then down at the page. Then looks at the pianist to make sure that she has ‘Zeffiretto’ correct, the pianist nods affirmatively.
Susanna writes down the next phrase, then looks up, and smiles out towards the audience as though she’s savoring the phrase “Questa sera spirerà.”
The Countess gets up and walks around the stage in reverie, and it makes it difficult for Susanna to hear. She leans in as though she’s trying to catch every word of ‘Sotto i pini del boschetto.’
She gestures to the pianist as though she needs to know what’s being said, the pianist shrugs. Susanna flags the Countess’s attention. The Countess is still off in reverie after acknowledging Susanna for a moment. Susanna catches the first half ‘Sotto il pini...’ can’t figure out what the second half was, then has an ‘a-ha!’ moment. ‘del boschetto.’
The Countess comes over to the pit, gives Susanna the hand, and helps Susanna out of the pit as she says “Ei già il resto capirà.” Susanna repeats affirmatively. They both then read over the document together and sing it again.
The Countess now takes the document from Susanna, and motions for Susanna to start singing it, as though she needs to commit it to memory. They take turns singing. When they get to the final apotheosis, the Countess takes Susanna’s hand, then both hands, then they merge in a hug. Then come out of the hug, and at the end, they giggle like schoolgirls at the ridiculousness of their prank.