Most of the talent that I've ever worked with are used to the routine and some even want to tell you that the sound guy on the other shoot had a lot of success hiding the mic "there". I've even had a few that just blurted out, "I'm not shy," and lifted the shirt or blouse the second the saw me approach with the lav & TX.
However, even with the most outgoing and cooperative talent, one slip and you're in uncomfortable land. Not a good place to be. The best you can do is be professional, keep a poker face, and get it done as quickly as possible and try not make readjustments.
I usually try to have someone from wardrobe there so that if I have to lift clothing or undo anything, that they can get talent back and ready asap. Then again most of the stuff I do is usually pretty small and I've never had to deal with more than 4 actors at once-- I really don't know how the reality guys can do it.