When I was an active photographer, it was very difficult taking candid shots of people at close range. I did not wish my subjects to be aware of my presence. It is much easier being invisible with a sketch pad than a camera, although I am careful not to stare.
Sketching is more stealthy to be sure, but as you can see details are more difficult to include. Faces are difficult enough to render, but on a small scale (8.5″x5.5″) with pen and ink, it seems impossible.
I concentrate on gestures and posture instead, at the moment.