Hiya!
(didn't read the whole thread...but here's my quick 2¢...)
We rarely use Insight without Deception. They are sort of "paired". I'm the DM and when I am RP'ing a NPC who is trying to Decieve the PC's, I *do not* call for any sort of roll. I just speak in character and let the players take it from there. Now, *if* a player becomes suspecious of what the NPC is saying, THEN I'll make a roll for the "Decieve" of the NPC and that becomes the TN for the PC's Insight. Pretty standard stuff I would guess. Now, the RESULT of that contested roll determines how much of an absolute answer I will give the Players.
DM: "No sir! I didn't see a thing! Honest! I was...uh...I was taking a leek out back when it happened. Missed the whole thing".
Player: "Uh, it took almost an hour for them to rob the place? You had to pee THAT badly? C'mon..."
DM: "Er, well, ok. I wasn't peeing. Fine. I was...engaged in activities with a street walker".
Player: "Really? What was her name? Where have you seen her? What did she look like?"
DM: "Oh, you know. A pretty lass. Her name was...Cerise. Yeah. Cerise"
Player I'm not buying what this guy is selling...I think he's lieing.
DM: [rolls Deception, gets a 9] "Roll an Insight please"
Player: [rolls Insight, gets a 17] "Last chance to come clean..."
DM: He is definitely lieing about most of it and blurts out "Ok! OK! I was drunk, alright? I was passed out drunk! I know I've been warned, but I really need this job!"
...and that's basically it. Until the RP'ing gets to the point where a contested roll is ACTUALLY needed, its fully up to the player to decide if his PC believes someone or not. This is likely due to my "old skool" style of DM'ing where the PLAYERS decisions have more effect on their success/failure in most situations (as opposed to a more 'modern' take of skill check first, then RP the result).
PS: I generally don't let a Player just blurt out "I'm going to make a [skill] check to see if...". So things like "I'm going to roll Perception to see if I notice anyone following us" isn't something I generally allow. I find it drags the game down too much in the "metagame" side of things. It also gives too much info to the player...info that the players character would have no real idea about if he rolls really high and I give a negative answer ("You got a 28? Nope. Nobody following you" <--effectively giving the player an "absolute" knowledge of the situation so he can quite reasonably expect he has time to do something critical that would be ruined if interrupted, for example).
^_^
Paul L. Ming