Menzel didn't put down his pencil when an acquaintance sat on the toilet. (http://illustrationart.blogspot.nl/2016/09/adolph-menzel-via-jim-gurney.html) He preferred to draw people unawares, often catching them in unflattering moments of eating, gossiping, or dozing. Once his friend Carl Johann Arnold awoke from a nap to find the artist busily drawing his portrait. “You just woke up 5 minutes too early,” Menzel told him. (http://gurneyjourney.blogspot.nl/search/label/Pencil%20Sketching)