Taking pictures of strangers can be quite uncomfortable at first. It reminds me of Richard Renaldi's project, Touching Strangers. This notion of taking a picture of a complete stranger is very out-of-the-box for most of us. I don't profess to know very much about the ins and outs of how to do it, but there are a few rules that help me when I go about photographing someone I don't know.
First, I (almost) never take the picture without my subject's consent. I do not want to feel like a paparazzi photographer stealing an image from someone to use for money. When we get our pictures taken, it's like a part of our soul leaves us and is imprinted on that film. I am not one for stealing people's souls. Basic introductions/consent is usually enough. If the moment is too good to wait for an introduction, or an introduction would ruin the setting, feel free to do it after you take the picture. You'll seem much less stalker-ish that way. When there is no common language, learn enough words to get your point across. This is what I did in China. When done this way, your subject can refuse you if they feel really uncomfortable (which is fine), or they will be OK with it; in which case, go ahead and shoot away! When I know my subject is OK with having their picture taken, I'm much more free to walk around, compose my shot, and maybe get in a few extra frames while I'm at it. I don't have to fumble around with being inconspicuous or quiet. Everyone wins!
Second, don't forget to compose your shot! Sometimes the anxiety of asking to take a stranger's photo gets you so excited and worked up that you forget the whole point of the exercise! I've definitely done this before. I had my subject, he was happy with getting his photo taken, and I blew it! The angles were off, the light was less than ideal, composition didn't fit, FAIL! It's OK to step back for a few seconds and make sure you know what you want in your shot before you take it. A stranger will be OK hanging out for those few extra seconds to ensure a good picture. They'll probably even be down with a little posing if you're *into that. *note Touching Strangers again.