You're right; just do it. Forget whether it's posed or not, you can worry about that later after you've got used to approaching people. I've done this quite a lot and in almost every encounter the person has ended up happy with having been asked so you're doing them a favour (I'm not kidding). Just be yourself, don't treat it as a big deal, be a bit cheeky and funny. If they ask you why you want to photograph them just say shrug your shoulders and say you're a photographer. It's very empowering to be able to use people you meet to help you out whether it's as a model or just to hold a flash for you.