IThe situation: A player creates a suave intelligent PC with great social skills and attributes (system doesnt really matter on this) because the player wants to play a character that is unlike her normal introverted self and is instead a dashing charismatic figure cutting a swathe through the social world of the campaign and always knowning the right thing to say to the angry duke, stupid chieftan, or winsome tavern wench/stable boy. However, said player lacks the ability to effectively role play such a character, because she lacks the ability to be suave and charming extemporaneously in game. So there is a tension between what the stats indicate the PC should be able to do and what the player can actually say the PC is doing. Leading to Frustration.
Tough one. No easy answers.
One of the things I like about RPGs is how they encourage people to enhance certain skill sets. In this case, the nice thing about RPing is it helps people get better at communicating. Likewise, to me the IC interaction, living out the story in an emersive manner, is at least 50% of the fun. So I'm going to lean on the side of doing nothing that discourages communication.
My preferred method with social skills is, let them roleplay. Try to evaluate the sense of what they have said as the hearer would, and decide how positively or negatively such an approach would be recieved
while taking care to not punish or reward the player too much for the delivery of those ideas.
With that later part, I might take into account the age, maturity, and charisma of the player. The more mature, charismatic, and skilled the player, the more you have to shine to get rewarded for being charismatic in your role play. I think you reward the player relative to what they are capable of.
Once I decide how good the pitch is, I assign some sort of circumstantial modifier. Then when the pitch is complete (which might take a few exchanges back and forth), I let them roll the dice and apply the circumstantial modifier. That way, if they put thier foot in thier mouth, coming from the highly charismatic character they are playing, they might seem droll, funny, affectionate, or force the NPC to see things from the characters point of view (just as we respond to criticism differently from people we like than from people we don't).
The problem may be though that this isn't good enough. Several things may still be going wrong. First, the player may be so abrasive or gaffe prone IRL that every time his character does anything, you are quite reasonably applying negative modifiers. Secondly, and maybe even more importantly, if the player is wanting to be charismatic, witty, and charming, and in fact isn't being charismatic, witty, and charming the whole process may not satisfying him (or anyone else) regardless of the outcomes the dice are producing. Simply put, a particularly uncharismatic person (or even simply an average charismatic person) may find a charismatic character unsatisfying because they aren't pulling off the role (and recognize that they aren't).
To this I can only say, not everyone can play every role. I can't. I know I've tried and failed with NPCs. There are roles where I have to be careful of, such as characters that are both highly extroverted and 'cool' that I usually can't pull off. (I'm generally happy with the outcome of cool introverts, bombastic villains, and extroverts with poor senses of propriaty.) So I try to disguise my lack of range by not introducing too many characters I'm not going to be able to pull off. I've never even tried to play a Bard as a PC.
If the player can't do the role he wants to do, then you've got another sort of problem - how to encourage them toward something they might enjoy more without insulting them.
