Okay, a practical example just came up in my campaign.
The party is a bunch of BIA agents who do X-Files-like cases (it's a d20 Modern game). They've been sent to a tiny town out on the Rez (reservation), and a whole mess of bikers just rode into town with guns out, preparing to threaten people and take money and laugh and be evil.
The party is there, as are a whole bunch of Hapless Villagers. Opening fire in this situation is gonna get a lot of villagers killed. The party is aware of this, and really unhappy about the situation.
The Charismatic face-man has just informed me that he won't be at the game tonight, but that he has a Bluff he wants to try. He mailed it to me:
Player said:
If the motorcycles are still making a good deal of noise, Stan
will try to throw in: "Did I mention that we were prepared for your shenanigans? The 194th Armored Batallion is lurking just over the ridge there. They have tanks and helicopter gunships and ordnance bigger than your head."
So now, as GM, I'm left trying to figure out what kind of bonus or penalty to give here.
Known Issues:
1) There is no military presence visible within the town. The party is wearing suits, not fatigues.
2) There's only one road into the town, and the lack of tank-treads should be, uh, somewhat obvious.
3) Helicopters make noise.
4) The idea that the tanks would lurk over a ridge, instead of sitting there in the middle of main street, is kind of silly, since, well, tactically, that's where the tanks would be. "Pinning the bad guys inside the town with a bunch of helpless villagers" ranks up there with "trapping the fox inside the henhouse" as a recipe for Pyrrhic Victory Stew.
5) The idea that the government would send tanks and helicopters out into the middle of nowhere to go after twenty bikers.
So, I am very comfortable putting this at a penalty. Right now, I'm trying to figure out if it's a –5, -10, or –20. My gut tells me that –20 is probably closest, because, in d20 Modern terms, this ranks up there with "Also, although I have no evident means of doing so, and we've just met, I've got your mother wired to a giant bomb in a basement somewhere in town, and unless you leave now, she'll blow up" in terms of plausibility. However, this player is really trying hard to roleplay a charismatic person (he usually plays low-Cha fighters or wizardly types in our D&D games), and I feel like slapping a –20 on him is pretty much just whacking him with the "No!" stick. With his scores, he COULD pull it off even with this penalty, but it'd take something close to a 20.
On the other hand, going by the flavor text, giving him just a –10 is pretty light. His roleplaying really hurt him here. If he'd said something a bit more believable, like "Well, these folks called us in, and we've got a few angry men with rifles in a few windows overlooking all of us, and if you don't turn around and ride out of town, it's going to be ugly for everyone," he'd be at a much lower penalty. A –10 penalty essentially means that an average dude will convince another average dude of this about a quarter of the time (two guys with Cha10, Wis10, and no ranks in bluff or sense motive, if my math is anywhere near right). Does "I have an army battalion waiting over yonder hill," sound like something the average guy is going to fall for one time out of four?
I might end up compromising at –15 (and of course, not telling the player this -- it's important that the players not always know the DC). I did tell him (via e-mail), "As long as you're aware that this is going to be a very difficult bluff check, that's fine." He's spending an Action Point on it, too.
Just thought I'd throw in a current-game example to see what people think. It's only sorta roleplaying, since the player isn't actually going to be there tonight – but it's what he'd roleplay if he were in fact there, and it's what I'm supposed to have his character say before rolling the bluff.