One of my little "habits" in covering events is that I would sometimes sneak up behind a photographer and take a photo of him/her taking a photo of a group of people, without him/her hardly noticing me at all. Another similar case is that I would take photos of people taking photos of themselves using their own DSLR or Point-and-Shoot cameras. Most of the time, they wouldn't really know that a "double exposure" was made until the photos are eventually shared to them by the clients via email, facebook, or the like.
I usually pull shots like this out of two simple reasons: 1.) a little story is made by capturing two interacting parties with their genuine expressions radiating in the photo, something that can be usually achieved if the photographer (in the photo) is someone dear and close to them, and 2.) if I need to take shots of interacting people, opportunities like this is one of the best ways for me to do so, without disturbing anyone in the process.
...which brings me to a little anecdote that I derived from a wedding that I attended, of which I came not as a photographer, but simply as a guest.
It is quite a relaxing experience for me whenever I would join wedding events as a mere guest, wearing some formal attire instead of my usual black polo shirt get-up. I get to sit back, relax, take a little time to appreciate piano music in the background as I look at the beautiful bridesmaids and their partners with their clean-cut suits, the adorable young ring and/or coin bearer that would usually zip from one end of the aisle to the other, the maid of honor and the best man, both presenting themselves at their, uh, honorably best, the bride and groom walking together towards the altar, and the wedding photographer that commanded them to stop in their tracks as he takes a shot.
Wait. What was the last one again?
Instantly, the feeling of relaxation went away as I saw what he did in the middle of the procession, which is basically part of the wedding event itself. He went from photographer to traffic officer and back as he took a barrage of shots with his "traffic-cam" at the "speeding offenders".
Why am I ranting like this?
In my humble opinion, a wedding photographer is fundamentally not at all, a part of the wedding event itself. He/she is there to take shots of the wedding, to cover the event itself, not to intrude or get in the way of anything or anyone, be it the actual participants like the officiating priest, bride and groom, or to disrupt the flow of the event in any way whatsoever. It is for this very reason photographers don black/dark attires, not flashy polos that will give their position away, or at least warrant unnecessary attention to them. (as if their battery-packed DSLRs with their 70-200mm. f/2.8 monsters aren't enough to turn heads around) If you need to ask anyone in the celebration to pose for a bit, do this only before or after the ceremony itself, not during the event itself, especially if it is something as sacred and solemn as a wedding mass. Of course, it also goes without saying that if for some reason, say, the bride and the groom suddenly stops in the middle of the procession to pose for a bit, then by all means shoot without hesitation. But make it very brief, very fluid, and as inconspicuous as possible. And don't even get me started about planning to say "one, two, three, smile!" on the said situation. That's a definite no-no.
It's an unspoken guide/rule: you're dressed up in black like a ninja, you might as well act like one. Move out, shoot, blend in. It's that simple. (and fun too, if you're into "role-playing", or something of the sort) :)