Well, I don't know how this would work... but why don't you try shocking them back to life?
Start writing the stupidest, most out-of-character stuff you can think of, and see if they don't come running back to set you straight on what they'd
really do.
And for me, when the characters get quiet, it's usually because I'm trying to push them where they don't want to go. So I have to review my whole plot to see if there's something off-kilter.
PJ