Except that, at least in extreme cases, he has a "tell": getting angry.
This is just my arm chair pyschologizing, but I don't think (as many do) that Clinton is completely, ah what's the word, anti-social, amoral? IOW he does understand to some degree the difference between right and wrong. Therefor when he "has to" lie he gets angry with however is "making" him lie, that, and the psychic energy required to convince himself of the "truth" or at least the "justness" of his lies, come out in anger.
IOW the anger is not feigned, nor part of Clinton "working himself up" to a lie, but rather is the result of the lying.
If he was a really good liar he'd be able to do it, even in an adversarial environment, and consistently keep his cool. But Clinton isn't that good a liar.
He gets angry in classic fits of narcissistic rage. Clinton wanted to play the great leader on the big stage. It was a vallidation for the Woodstock-ME generation which he is the poster boy. Now that history is showing him to be a dangerously naive, self obsessed buffoon with no self control- He explodes with rage. We have seen behind the curtain.