Writers are nosy. We eavesdrop. We watch. We take notes. Then we use whatever information we glean. I don't mean we necessarily take a ... well, say a conversation between an arguing couple, and put it on the page exactly. I mean we use that moment--a moment that certainly doesn't belong to us, but we claim anyway--to create something new or to improve something already there in our work. We remember facial expressions, hand gestures, tone of voice, the look in someone's eyes as they whisper or they laugh. It's the well that we draw upon when we write about our characters.
It's not always a conscious process. Our ears and eyes and minds are open to the experiences unfolding around us. Granted, I know that for myself, I can be so interested in what other people are doing, I forget to pay attention to what I'm supposed to be doing (or, y'know, that I'm with a person who wants to talk to me and not watch me stare in fascination at the people around us ... what?). The thing about being an observer, about being someone who's constantly in her own head, thinking about stories or dreaming about traveling or wondering what that girl over there is feeling as she shovels ice cream into her mouth (Did she break up with someone? Does she just REALLY like vanilla? Is she gonna finish that whole thing?) ... um, well, you can see the problem.
If we're really lucky, we find in the actions of others a plot twist, a character flaw, a sudden realization for solving a problem in Chapter 12, or a snippet of something we haven't quite defined, but we save for later. Being all up in someone else's business (quietly and indirectly, of course) is only one way writers fill the well. Sometimes, we steal from the people we know ... and by "steal," I mean "appropriate for creative purposes." A friend's quirk or catch word, something funny said in conversation, an incident that becomes an inside joke, a look or gesture ... these things are imprinted along with other observations. Sometimes, we use them on purpose, but mostly, I think they enter the flow of our writing on their own.
I enjoy being alone in a quiet room with just my computer and my thoughts (and my puppies for emergency snuggles). I can spend hours--hell, days--on my own without talking to or looking at another human being. But eventually I seek out connection. Sometimes, it's just being with a good friend, and sometimes it's going out among the masses. Sure, it's about being counted among them, even though I don't usually feel part of the crowd. I'm on the outside of it all, waiting ... and watching.