I always carry a moleskine sketchbook with me to write down ideas or to make a note of things I see and like while I am walking around. They are often a great resource for Illustrations and ideas for films and animations.
I also use them to sketch obsessively and over the years I have collected dozens of sketchbooks full of drawings that are stacking up in my office. I have noticed that they are mainly heads of people I see while commuting. They are not really portraits, I tend to use the drawings as a way of dissecting what I am seeing (...an unusually long neck, the shape of a head or the direction of a person's hair and their double hair crown). So most of the drawings are really fast sketches on shaky trains. Still, I really like some of them, for different reasons, the mark making, the simplification of a shape or the looseness of the line.
These are a sample of those drawings a quick trawl through a few sketchbooks.