There's a decision fork here, and it's whether to mirror the movements with respect to Frisk, or to press the exact same keys. With wild abandon, Chara chooses the latter, and adds two extra notes to the end, chosen entirely at random - it isn't really a melody, although it doesn't sound bad.

There's occasional flicks of the eyes towards the other, because it's rude to stare but Chara needs to keep a bead on the other's disposition, but their smile, while somewhat manic, isn't edged with extreme stress.

(Just normal levels of this-Chara stress.)