Another in-game tactic I suggest is that you basically dare the npc to kill off the group. The group says, "No, we won't do what you want. You tell us what the deal is or kill us. You obviously need us for something or you would have done it yourself. So choose: kill us, tell us what you want, or leave us alone."
If the npc kills everyone, well, problem solved. Time to discuss with the DM just what exactly he was trying to accomplish with this guy.
If he talks, then negotiations, deals could be made, etc.
If he leaves you alone would be a success.
Lastly, the npc begins tossing the Charm/Mind Control/Do What I Want You To Do spell and the PCs do it anyway. Time to discuss with the DM just what exactly he was trying to accomplish with this guy.
Otherwise, as I posted earlier:
"If the npc wants you to do something then think of it this way, your group is being coerced by a more powerful person. Unfortunately, you've got to obey until you can figure a way to turn the tables, escape, or kill him."
I still believe in-game solutions are always better than meta-game ones.