This wonderful painting is a superb example of creating the feeling of depth or space with a plane.
In this painting Giacometti provided a plane, or shape, appearing in front of the face. This is an consideration coming directly from the influence of Cezanne.
Now carefully peruse the right side (his right) of his head. Do you see a straight line at the edge of his hair, running down through his ear to the edge of his neck down to the collar? It then takes a right angle turn to the bottom to his chin, creating the bottom edge of the plane. Now go back the where we began in his hair and note the angle which defines the top of the plane. It’s subtle until we notice that wonderful vertical line in his hair on the other side of his face.
Keeping in mind that the shape is meant to be felt, rather than seen, do you see the plane in front of his face?
You may feel his face projecting through the plane, which is fine, as it is meant to be open for interpretation, as there is no right or wrong way of seeing these considerations.