Starfox
Hero
To sum up the link: Insight is global (about the entire demeanor of an NPC), not local (rolled for each and any phrase uttered that might be a lie). Here's an example:
DM: Your NPC companion points you thru the tangle of buildings in the thieves' district to a dark alleyway with a single door lit by a blue lantern at the end.
Player: Is the NPC hiding something? *Rolls really high*
DM: The NPC keeps looking over his shoulder like he is nervous or expecting to be back stabbed at any moment. He has been very cooperative, but his words have been terse, like he is revealing only precisely what you ask.
Notice the DM did not say "yes, he's lying to you."
Now it's up to the players to converse more with the NPC, threaten him, or use what they already know to decide whether the NPC's jitteriness is because he is being coerced by the party to guide them, because the NPC has bad history with the thieves' guild, or because the NPC is setting them up for an ambush.
While this is fine and dandy, it also opens up a whole different can of worms - how much should the player be forced to read the DM, and how much are we measuring the player's social skills as opposed to the character's. If I gave the above response to a player with poor social skills, I might get "please answer the original question" back - and I think that is a fair comment. Just as a player cannot improvise every word of his character's Bluff, he cannot be expected to read every nuance of the DMs language.
In the situation above, I feel that a mediocre roll should yield the information given in the example, while a good roll should be more direct and focused. If the conversation is actually acted out, the information above is what anyone should be able to infer from how the DM role-plays the guide, while a character with a good skill roll should gain more. But I know this varies widely from table to table - how much information to give out and how is a big part of DMing style.