The first step is to make him feel secure.
“What’s your Marshguard name?” you ask.
He doesn’t answer.
“Don’t want to say? Fair enough, I know plenty of Marshguards are stingy about revealing their nicknames.”
At “stingy”, you notice his head turn slightly, and that his free arm tenses up a bit. Hmm. Perhaps he had trouble getting money out of someone in the past? It’s not an unusual story for Marshguards.
Or, it could be the language barrier. He might have trouble telling the difference between “stingy” and “sting”. Doesn’t like insects? Or maybe his name relates to one.
“Well, for my part, I’m not afraid to share my name. It’s Lisa Black. And I just thought I’d warn you, I doubt the base will release our prisoners. They don’t tend to take kindly to active hostage situations.”
“You trade,” he snaps back. “You trade before.”
“Not under duress like this. You’ve got his life in your hands – that’s very different from keeping him in a cell and feeding him. Besides, even if they did relent, they’re not going to release more than one prisoner.”
“Two prisoners not so much more than one.”
That catches your attention. He knows you only have two prisoners – that must mean he knows Mudviper’s escaped.
Is there anything useful that tells you?
He’s not alone.
The “stingy” line is a bit of a stretch, but Lisa’s unusually lucky. I don’t think I would have gone for it with a different viewpoint character, though.