Flying is like dancing. River gets the same rush of adrenaline in her veins when she sits in the pilot's chair that she used to get at ballet practice. Learning to bank, to pitch and roll, to dive and to pull up just short of certain fiery death is just like perfecting her turnout, or mastering Odile's thirty-two fouettés.
It's the same feeling of freedom, too, the joy of being lifted up out of her skin, when she closes her eyes and feels Serenity vibrate under her hands.
The day Mal says, "And this here is our genius pilot," to the passengers they're ferrying from Boros to Greanleaf, she feels that joy bubble up and over, takes his hand and twirls, toes pointed and skirts flying out behind her like Serenity's vapor trail, a dancer amid the stars.