What's the difference between the Investigator's discern lies ability and the normal use of the Sense Motive skill?
The Sense Motive skill can be used for two purposes: to overcome a bluff, and to get a sense of a GM character's trustworthiness. Understanding the difference between this skill and the discern lie class feature begins with understanding the difference between a bluff and a lie.
A lie is a simple misrepresentation of the facts. For example, if a suspect tells you she was in Chicago on the day of the crime, when in fact she wasn't, that's a lie. If a client tells you she'll pay $10,000 for the job when she really intends to stiff you, that's a lie too. Body language and attitude are part of such communication, but not necessarily the major part. A lie may be very sophisticated and well thought-out, and it is intended to deceive someone at least until evidence to the contrary is discovered.
A bluff, on the other hand, is a quick prevarication intended to distract, confuse, or mislead someone -- generally only for the short term. It is intended to momentarily deter an action or decision, not to withstand long-term or careful scrutiny. You bluff your way past a security guard by flashing your video club card as though it were a press pass and acting like you know what you're doing. You bluff your way out of a brawl by acting like you're tougher than the 250-pound biker who's challenging you. Bluffs depend almost completely on attitude and body language. They may or may not involve actual lies, but if they do, those lies usually aren't very sophisticated and aren't intended to deceive the target for more than a few moments.
The first usage of the Sense Motive skill mentioned above allows you to see through a bluff with a successful check, but it doesn't help you determine whether any given statement is a lie. From a purely mechanical standpoint, an NPC should not have to make a Bluff check every time she utters a lie. And if no Bluff check is required, there's nothing for the Sense Motive check to oppose.
The second function of the Sense Motive skill is to determine the general trustworthiness of a character. When the skill is used this way, a successful check might reveal that your suspect is highly -- well -- suspicious, and that she might tend to lie to you. But that doesn't tell you which, if any, of her statements are actually untrue. In fact, an NPC can be highly untrustworthy even if she doesn't happen to be telling any lies at the moment. Again from a mechanical standpoint, this usage of Sense Motive requires a whole minute to use, so it can't be applied to a single statement.
The discern lies class feature, on the other hand, is specifically designed to determine the truth of an individual statement.
