I often get emails from people who've seen my train drawings, asking for advice on how to go about sketching strangers in public.
Firstly, I never, ever ask permission: if you do that, you're stuck drawing a 'portrait', which is a totally different thing; people are no longer natural, plus there's suddenly an expectation on you to achieve a likeness (as well as a need to make sure it's flattering!), which makes things quite stressful.
My 'victim' sometimes works it out halfway through, but then they tend to hold the original position and carry on with what they were doing.
There are tricks: firstly, I have found trains are perfect - when people are travelling, they have less objection than when they're 'at leisure'. It's dead time, so somehow up for grabs. I know sketchers who've been challenged for drawing strangers in a pub or restaurant: people can feel you're invading their privacy in these places.
I plucked up courage to try sketching at the hairdressers once:
Museums, galleries, cafes and queues seem to be other universally acceptable venues. Buses are too bumpy by the way, and people get off to soon.
I always choose a table seat on the train. It gives you a better view of more people. The main reason though, is I can put my handbag on the table and my sketchbook in my lap, so it's hard for anyone to work out what I'm up to.
I try not to draw people with friends: conversation makes them move constantly. People reading books are good, but newspapers are a problem: too many different head positions as they look at different articles! People sleeping, texting or at laptops are the best of all - total absorption, so they rarely see you, and they hold one position for ages.
I sometimes show people, if I'm proud of the drawing. Also, if they realised I was drawing them, it's a nice courtesy. I've had some lovely conversations as a result: it's a great way of bonding with complete strangers in a weirdly random way.
People's reaction generally ranges from disbelief or surprise, to feeling flattered. Non-plussed is the most negative I've had so far.
If you are new to it, I suggest you focus on particular details - shoes, hands, bits of faces - rather than going for the whole person.
Go for venues where folks are likely to be sitting for a while. I tried the forecourt of a station once - hopeless: everyone was in a hurry and very twitchy!
Try not to rub out - it makes a mess and takes up valuable time. If it goes wrong, either draw over the top, or move to a new bit of the page. Also, have 2 or 3 sketches of the same person going at once on the same page: that way you can jump between them as they move about:
You can view a selection from my various sketchbooks on my website and I have created various short films about how I use my sketchbooks, which might be useful in getting you inspired.
My book Sketching People, goes into a lot more depth on all the above, and much much more. If you would be interested in attending a sketching workshop in person, please sign up to my mailing list, so you get all the information of what events I'm running and first refusal on places available.