I'm pretty sure once you talk to an NPC, everyone nearby will be alerted regardless, so I think having the guards around at the time would be a mistake, since (IMO) it's not gonna work.
So... what if you include an objective where you have to distract the guards? You could even do the classic "http://tvtropes.org/pmwiki/pmwiki.php/Main/MuggedForDisguise" thing, where the player ambushes an enemy and takes their clothes (using a New Vegas-style faction-changing outfit). Then, once disguised as the enemy, it's just a matter of going up to a guard and telling them that the boss wants to speak to them or something. You could even throw in a Speech check for some added challenge.
Then, once you're alone with the prisoner, you can do whatever it is you need to do for your quest.