Surreptitiously, River studies him. On a job his eyes are flint, able to cut, to burn. But when the job's done, when he's on Serenity, they dance - like light meeting water. Best is his laugh, happiness fanning 'round his eyes.
Mal does likewise - scrutinizing her, that is. Nowhere near as stealthy. But he'd not be receptive to her noticing back. So she acts unaware, working each bit of new data into her equation, recalculating his readiness.
Not yet, but soon he'll know: she is steel to his flint, water to his light. They will spark and dance together.