My two esteemed colleagues have already said a lot of what I was thinking...
... but that won't stop me!
The 'tell them in advance' technique is good if you can get hold of them. When I was
doing radio OBs in 'production house' mode there were generally four different
1) It's not going to muck-up our system is it? (Usually from those who
were willing volunteers who didn't know too much about live-sound.)
2) Sure, go ahead
as long as you've got all the extra cables needed.
3) OK; but keep it well out of the
way of our gear - on-stage or by FoH.
4) He's from the BBC so it must be alright! (I
wasn't; and not necessarily so!)
There was also sometimes a worry about two
lots of phantom from those who didn't know how such splitters work. That said, I usually
negotiated an agreement on who would supply phantom to the mics.
The only other
thing I'd say is that even if you've secured previous agreement it's good to get
everything connected-up before soundcheck - especially if you'll be working with those who
aren't quite sure about what you're doing anyway.