The blogger Bowl of Orangeshas a post about "the thin line between being a pervert and being a writer." In the post, Bowl of Oranges (BOO) confesses to being an eavesdropper. I'd equated eavesdropping with being impolite but not with perversion, but I take BOO's point: writers do like to stare, overhear, smell, touch, and taste things to see if these things might just fit in with some writing or even inspire words.
I tend to listen in when the people talking are talking loud enough for others to hear. I think of it as a free broadcast. I don't ever move closer to get in better listening range. (I think I do probably stare at people too much, absorbing details, but almost always when they're not looking, so that I'm not perceived as being rude; nonetheless, I probably migrate over the "polite" line, something I need to watch, so to speak>)
What I do more often is stay receptive to what's said by people walking past on the street or waiting in line, and in a big city like Istanbul, that happens all the time--but happens frequently on a small college campus, for instance, or in a store.
For example, a few days ago, to women were walking by slowly, and I heard a snippet of their conversation. I deduced that neither was British, Canadian, or American but were mostly likely also from two different countries so that English for them as a compromise language. In any event, one of them said, with her particular was of accenting English, "Life is so unexpected."
--It is indeed, as are such bits of language for which one may scavenge as they are spoken into the air and then lost unless one captures them. What to do with the language?--well, that's all about a writer's choices. At the very least: savor them. I mean, "Life if so unexpected": what a great sentence, so unexpected.