Still a wee bit confused on deceit. Is it alright if we work through an example?
Let's go with something basic: the person they are talking to is claiming they dont know anything when really they do.
1.) The pc roll empathy and the Npc rolls deceit in defence, modified by rapport.
-Failure!: The PC sees through the deception and understand that the NPC knows something
-Success!: The PC has no idea that they are being lied to [but the player does, since the NPC rolled deceit and beat them]
2.) The NPC rolls deceit as an attack to utterly fool the PC into thinking they are innocent and know nothing. The PC rolls empathy in defence. (Could rapport work here?)
-Failure!: In trying to push the falsehood too far the NPC reveals themsevles. (any aspects or consequences appropraite here?)
-Success!: The NPC places the consequence "Is sure of my innocence" on the PC. (Perhaps compells it to make them stop asking him questions?)
Also, could you just use Deceit to perform a manuever to place and then compell an aspect in a similar manner to example 2's success?
One of the ways I handle failure on deceit rolls, is to let the person looking for information get a little bit of it. Probably not the information that he is looking for, but at least a tid bit that he can use to represent his character knowing the other guy is lying.
Let me expand your scenario to give a broader picture of the Deceit exchange: PC (P1) and NPC (P2) are at a police station. P1 wants to get information about a crime out of P2, and P2 wants to conceal the fact that he was involved from P1. Notice that they have already performed the crucial task of deciding what winning would look like to them. To P1, winning looks like P2 giving important information up, and to P2, winning looks like concealing his guilt. There is even enough overlap between goals that interesting concessions are possible.
Exchange 1:
P1 tries rapport to get P2 to talk. P2 beats it with rapport, so he talks, but not about anything important or helpful.
P2 tries to convince P1 he knows nothing with a Deceit attack. P1 gets hit, but not enough to take consequences, so the deceit seems honest, but not enough to get P1 to give up the questioning.
Exchange 2:
P1 uses a rapport assessment to see if P2 is really as ignorant as he is claiming (GM rules this use to be acceptable). P2 resists using Deceit, and fails. So the GM lets P1 know that P2 is lying about what he knows, but not what, and inserts an inconsistency into P2's dialogue, along with revealing P2's Slippery Scum aspect to P1.
P2, not wanting to get further entangled in his disavowal's, decides to try a new tactic. This time, he says that he knows for certain Big Tom was involved with the crime, but he was too scared of reprisal to admit it earlier. P2 has the true lies stunt, and uses it to boost his attack (because Big Tom, apparently, was involved). P1 get hit hard, and takes a consequence of "Uncertain".
Exchange 3:
P1 doesn't want to take any more consequences in this fight, so he offers a concession. He'll let P2 go, but will still suspect him if the Big Tom lead doesn't pay out. P2 agrees, and thats that.