Because we are so good at recognising small differences between people it is very difficult to bring out the human form convincingly.
When we painted Freud's ear we used well defined values of a hue to create the illusion of form. On Friday we are going to continue that journey as we start with bold blocking and then move towards more subtle value gradations.
From this detail of a Velazquez portrait we can see how the nose was carefully constructed to bring out the bone structure, and as we look at great portraits and figure painting we realise that every part is built up like a geometrical structure so that we are deceived into thinking it "looks natural".
That process is hard to detect when subtle changes in hue and value look seamless in areas such as the back or legs. The use of small hatching strokes can provide good control to define how light reveals the planes of figure such as in this Degas pastel.
However we achieve it, the goal is to create "sculpted" shapes in paint by the simplest means, so that a likeness in paint begins to be a matter of process rather than luck!