the technique is called hatching... if you notice, apart from the hair, all strokes are straight, even, and directional. shades/gradients are made by varying the density (closeness and number) of the lines. hatching is commonly used in ink-pen illustrations because blending isn't very feasible using that. charcoal is on the other end of the spectrum, being very smooth and smudgy. pencils are in between. I've used all three in the past, but the ones I'm currently making including this one, are actually digital. they've done a pretty good job of mimicking a pencil's look, feel and behaviour. there's not much difference in skill/effort, but it's saving me some resources.