There is something that happens when a glass pane separates you from your subject.
You suddenly get much closer than you would probably otherwise do, as if that physical barrier made the approach less invasive, providing a kind of shelter both to subjects and photographer.
Most times, actually, people keep doing what they are doing, oblivious to being shot. Reading, writing, eating.
And when they notice, it's a startled look that suddenly lights up their face. Deprived of verbal interaction, the are left with their eyes to say it all...