One un-noticed feature of Rembrandt's painting is the pose. Hendrickje leans against the door like Rembrandt would have raised his brush-hand to paint hers. The edge of the door, moreover, is just visible in the original like the edge of a canvas. Max Liebermann in a 1908 self-portrait adopts a similar pose, his head at the same angle. He, however, is right-handed but Hendrickje, the mirror-image of her right-handed lover, is shown as a lefty looking back out at Rembrandt to "paint" him.
Note, too, how her hand is level with the diagonal of her eyes. He thereby conveys the link between hand and eye that had long been the hallmark of great art.