Rarity loses herself in the music for a bit, letting the notes fly from her bow and strings. It's helping with that itch in her fingers - it's not assuaged it completely, no, but it's helping.

And then she catches Marty's look, and his meaning, and sends a very flat one his way in return. If he wants vocals, then they're heading into more familiar territory, because she is not improvising. Gradually, she eases her playing into a new tune, looking to Kokoro as she does. This will require a tempo change, and it won't work if he's not on board.