It seems my eavesdropping hobby is not as creepy as my friends think! In "How I Write", author Janet Evanovich, who creates believable and endearing characters, says she eavesdrops for character development (p.5). This is a great little book with keen insights on the writing process from characters to plots to publishing. I highly recommend it.
Don't worry, I'm not using electronic listening devices or slinking under window sills yet. Just putting my two big ears to use and sharpening observation skills. I do carry a tiny voice recorder that I whip out instantly to log details about people I notice. It's amazing how much someone's appearance and mannerisms can suggest things if you're paying close attention.