John Lennon

I've been visiting this piece over and over, and i can't believe how well you've drawn all of the different textures, light reffractions and shading.. It's unrealisticly realistic for a graphite drawing. I realise that years of hard work have paid off in terms of experience and eye to hand coordination, but forgive me for asking, from what were you able to learn about human facial expressions, and generaly, about the technical aspect of drawing portraits the most?
All the best in your work sir!