Mrs Gaskell has left some of her notes for us to study, too.
::rapt attention::
Now, in Mr. Thornton's face the straight brows fell low over the clear, deep-set earnest eyes, which, without being unpleasantly sharp, seemed intent enough to penetrate into the very heart and core of what he was looking at. The lines in the face were few but firm, as if they were carved in marble, and lay principally about the lips, which were slightly compressed over a set of teeth so faultless and beautiful as to give the effect of sudden sunlight when the rare bright smile, coming in an instant and shining out of the eyes, changed the whole look from the severe and resolved expression of a man ready to do and dare everything, to the keen honest enjoyment of the moment, which is seldom shown so fearlessly and instantaneously except by children.