This illustration was a concept for a client of mine. The rough concept illustrations started on paper but once I got to the point where it needed details I would draw just a single side of the face, knowing that I would reflect it once I got into Illustrator (just the easiest for me).
As any designer/illustrator understands knows when you bring a scan into Illustrator and begin drawing, it can sometimes be difficult to visualize the final illustration, not to mention some techniques work better on paper vs the computer so you make some on-the-fly adjustments to your drawing.
I found by applying appearance attributes to my layers, it was much easier to draw and view the face I was creating in real time. For example, 1/2 of a nose may look great on screen, but after you spend precious time working out the details of the half and finally reflect it, it may appear too narrow too wide or whatever. When using the appearance attributes, you get to see your changes real-time without drawing, reflecting, then deleting and so on.
I picked up the technique from this video. It’s worth a watch to understand the process of setting up your layers in Illustrator.