Example: notice how we clearly see David, and David's eyes, nose, cheeks, mouth, hair. However, notice how this isn't really the case, we see a bunch of shades of gray.

If you wanted to paint using this model you would try to first observe, REALLY LOOK at his face and try to notice what stands out. What characterizes him to you? What would you ABSOLUTELY need in making him look like him (since you have limited time, as you're in the transport).

After that you would attempt to add lines or strokes to your page while observing your model, looking at the page only to calibrate and execute strokes.

The process is intuitive, takes time to develop, but IF you can laugh at yourself, it's pretty fun throughout, ELSE it will be painful at the beggining, cause (at least for me) my first tries looked terrible, but with time, they looked less terrible and so on.