You are allowed to pretend you know more about them than you do.
You are allowed to lead them to believe you are from another country.
You can do good cop/bad cop, which implies that you can pretend to be angry or unreasonable when in fact you are not.
This really bums me out. You all didn't know it, but I have been a private contract interrogator with the CIA for a number of years. I'm probably going to have to be done with that now, because throwing around lies was easily my most reliable method of getting information from subjects. Herein you will find some of the lies I've used to great effect, but which are now illegal:
The Lie: "I checked the messages, and no one called for you."
The Goal: To break the subject's spirit by implying the no one cares about them.
The Truth: The terrorist in question had four messages, although I think one was some random survey thing calling. I deleted all four. Just. Like. That.
The Lie: "Me [sic] and the other interrogators were going to go play Wii bowling off base tonight and wanted to ask you along. The Colonel said no. We really wanted you to go though."
The Goal: Good cop/bad cop, using the Colonel as the bad cop in absentia. Using this technique, the subject desperately wants to join the group (we only let them play XBox in their cells [duh, not XBox 360]), and gives up information to try to feel "in." GC/BC is still allowed, but this takes it too far in the clear infliction of emotional distress.
The Truth: We never had any intention of having him go along for Wii bowling. The truth just hurts sometimes.
The Lie: "I texted you like three times, honest. You never got them? I figured you were just ignoring me."
The Goal: Make the subject worry that their wireless provider has bad service and/or poor coverage in the cell block.
The Truth: I had told the subject I would text him after lunch, but thought better of it later. I never texted him even once. Well, I did, but I hit cancel right in the middle of the send, and I'm pretty sure it stopped it from going out. Sometimes we follow it up with "Everyone in D block is getting my texts, so I'm not sure what's up with yours." This breeds jealousy and distrust among the detainee population, a valuable tool.
The Lie: "I saw your sister at a bar last weekend, and she was acting totally slutty."
The Goal: Shake the subject's faith in his family's virtue.
The Sad Truth: We try to track all of our detainee's families in their native countries. His sister hadn't even been seen in the last few weeks, after her appearance showing waaaaay too much ankle in the "Taliban Gone Wild '09" calendar. One barely has to imagine what happened to her after exhibiting such decadent behavior in public.
The Lie: "Guest what? Chicken butt."
The Goal: This one's insidious, which is probably why it's not on the approved list. The subject thinks that something awesome is about to happen. Maybe it's their birthday, and they're expecting cake. Maybe they think we're going to let them go. I don't know how these crazy people think, so I'm not sure what depraved wish enters their head when I ask "Guess what?" But I do know that it warms my heart when they excitedly answer "What?" and I crush their spirit with "Chicken butt." It gets me every time.
The Truth: There is no truth to this one. I think that's what hurts them the most. The entire question is a lie.
Without the ability to administer these kinds of mind-warping lies, I'm going to have to tender my letter of resignation.