See the post for continued comments.
Left: An 8 x 8 inch (approx.) page from a journal I made with defunct drawing paper. Waiting room sketch—Staedtler Pigment Liner with light washes of watercolor (Daniel Smith). (Thick book and paper buckling causing shadow when scanned.)
I take my journal with me everywhere and this is useful because there are life models available everywhere. I sketched this man while I waited the 30 minutes required after my allergy shots (they want you to be on site if you have an allergic reaction).
On this day, the two people sitting next to me, who were too close for me to sketch, noticed what I was doing and we ended up having a very interesting discusion about cat adoption, dogs, and life.
In March I started a series on Direct Sketching with Pen and Ink. If you are always working in pencil and enjoy pencil, that's great, but if you are working in pencil and constantly erasing, I would like to encourage you to pick up a pen and have at it, no erasing. My first post in the series walks you through my approach to sketching in public.
On this day the man I was sketching was actually in separate waiting room which I could see from the smaller waiting room I occupied. It's my experience that people generally are unobservant and specifically only look at their immediate surroundings. If you can position yourself in an adjacent room that looks on to their location, or even a different bank of chairs (as I did on my jury duty sketching adventure), or a different compartment in a train, they won't even realize you are focusing on them. Try it.