I usually turn the flash way down so it acts as micro fill light and try and get someone to hold it from way back so the colors are saturated and the the mood isn't changed. I actually set the flash in one of these pics on a bar with a bottle in front as a diffuser. These are just one of those shots you get when you get lucky, when the mood is just right. But I must add that I am a post processing junkie. At least in my part of the world (Bolivia) you don't have to ask for permission to photograph someone. Thankfully we still don't have those problems. Plus people have gotten used to me packing a camera lots of the time. My D3s is a great camera, but lately I have been playing with a Leica for it's unobtrusiveness. I just can't break the confidence barrier with it yet and it has disastrous noise low light.
“ Your first 10000 photographs are your worst." – Henri Cartier-Bresson