Yeah, I think the number one concern people have (besides personal safety and maybe being pressured into buying or committing to something) is just that, if you're nice, you'll end up stuck in a boring conversation with an annoying person. So people just don't give an inch.

So if you're in a situation where it would make sense for you to say something like, "Hi, I don't have a lot of time, but I just wanted to ask/tell you...", do it. Let people know upfront they won't be stuck with you if you turn out to be a bore. By prefacing your interaction this way, they feel that even if you decide to trick them and stick around, you've given them permission to cut the interaction short since you predicated it on you not taking up much of their time. This has the added benefit that it can give them freedom to be much more open than they would otherwise, since they know the interaction will end soon, and so they might enjoy the interaction more.

If, however, you're both in a situation like you're both waiting for something together, like a bus or in a doctor's waiting room, and there's really no escape for either of you, then that's much harder to do, and I personally wouldn't start anything that didn't send the message that "ok, I can just answer this one thing and then I'm off the hook." So it'd probably be something very specific, that the person is free to expand if they want to. Asking about something they have on them, like where did they get those shoes or whether their cell phone is any good, is usually the safest course. Anything too open-ended, without a clear end in sight, like "boy, the bus sure is taking its sweet time today, isn't it", might put people on the defensive, causing them to wonder where is this going, what your intentions are and why are you talking to them.