It all starts with choosing the defining lines for each particular piece. In the case of The Joker, that would be his wicked smile that defines the cheekbones, the lips, the outline of the nose, eyes and ears. Those are drawn using a custom line brush in Illustrator.
From there, internal lines are added at about 1/2 to 1/3 width to fill the remaining areas. The blend tool was used to create transition lines, but sometimes those need to be altered, depending on the area. All together- it’s a whole lot of MF lines.