How does this work when an NPC actually does lie to a PC? I'm gathering that this can still happen. Is it just about player skill here?
"The deranged wizard says, 'Your companion never came to my tower; I've never heard of the man.' "
The player then either says, "I don't believe him," or, "I guess we're at the wrong deranged wizard's house."
It's up to the players to accept things as the GM presents them or not - is that how it's handled?
i can't speak for Umbran, but I agree with his premise that the skills are for detecting NPC lies and for manipulating NPCs.
So if the NPC lies, the player might doubt it and want to check. If he says he doubts it, I roll a check. If he doesn't say a word and believes him, the PC has voluntarily failed his save.
If I was on the ball, I could always roll a Sense Motive check on every statement I narrate, and give the NPC's statement followed by a "you think he's lying" whenever the roll suceeds and the NPC is lying.
For PCs, because people have a tendency to secretly resist against any decision they did not win, it's safer to not use a die roll (the decider) to force a PC to believe a lie that the player knows is false.
So when an NPC lies to a player, the player can decide to believe it or not, and the player has a safety net of the skill check. But the player is still free to decide to believe it or not (even if the check says "it sounds true").
When a player lies to an NPC, I can just roll a Sense Motive for the NPC and let that validate if he believes it. I don't have to get involved as the GM except in abjudicating it and applying modifiers for good or crappy lies.
When a player lies to another player, you have a new level of deciept and belief going on. If the PC tells a lie that the target player knows is false, he's not going to believe it. And he's going to rebel in small ways with his PC that he has to "accept" this fact if we enforce a dice roll check that says the PC is gullible enough to believe it.
So, I feel we're better off letting the player decide if he believes the statement, and playing his PC to match his decision.
As far as role playing it, I prefer the player to make the statement in character. A high CHA character will be charming. A low CHA character will be blunt. Then I roll the NPCs reaction behind the screen. If the PC succeeds, I play out a favorable response. If it fails, I play out an believable negative response. I don't mangle a player's speech from a high CHA character that was well delivered but rolled poorly. It is still quite plausible for the NPC to say "I hear your proposal, but I'm still not inclined to agree."
Generally, my group does not handle a social skill check without a role-playing statement. "I bluff the guard to let us in" is not sufficient for me to grant a check or permit access. The player will have to at least explain what he's going to say, if not actually say what the PC would say.
For a less important social skill check, like buying some supplies as part of the pre-quest stock-up, and hoping to get a discount through social skill haggling, I might just roll the skill check and apply a discount (or penalty if the PC rolled bad) as part of the take the price from the PH policy for shopping without wasting the GMs time.