There are some players that routinely irritate and frustrate all the NPCs around them. Their PCs are constantly on the receiving end of the stinkeye and may, by their way they handle interactions, negate any and all character-based charisma that they might have going for them. I think I've already said before that I like to apply charisma (and other character stats) as a sort of "passive lens," that is if a character is meant to be great at oratory and the player is not, his crappy orations are altered at some point after they leave his mouth but before they enter the world (via the DM, that is, me).
This helps players who have what we might call middling or low charisma to be treated as though they had higher charisma by the NPCs their PC interacts with. However, there seems to be a threshold beyond which players cannot recover, a social awkwardness that simply cannot be overcome by any kind fo passive interpretation. The leading elements here seem to be intense weirdness/morbidity and high-handedness with NPCs.
Many players fresh from the 3x world and beyond (I have noticed) tend to believe that an important or powerful NPC will appear to be important and/or powerful; these visual cues allow them to know if they need to treat them with deference or not. Perhaps it's AD&D and perhaps its just me, but I have always felt that that need not necessarily be true. Weak NPCs can dress up in fancy clothes and look like dangerous knights (though of course, attentive players will notice that they do not carry themselves like men who know what they're doing). High level NPCs may appear to be grubby, ill-kempt, or otherwise unassuming.
Even veteran players from AD&D have made the mistake in the past of treating important/dangerous/angry NPCs as though they are shitheels. Even maltreating barkeeps has been known to have its effects (saddling a currently running party with the unfortunate moniker of "the Crabs," a tale in and of itself) and so I always recommend that players cultivate a healthy sense of humility in their first few levels. Of course, if you're intentionally playing a snooty fellow, that might be something different altogether and there are, of course, cases where one can be imperious but also powerfully charismatic. Not that this approach will apply positively to all people in all places, of course, as charisma is possibly the most subjective of the core stats.
The combination of player action and PC stats creates what I like to call an "effective charisma." While a PC may have a charisma of 16, if the player has a real world problem with awkwardness they may wind up having an effective charisma of 8 or less. I know this penalizes people who have no social skills. I know this penalizes people who can't act or at the very least pretend to have social skills. But D&D is a social activity, and if you can't pass for at least a normal charisma you can't play a character who can talk his way out of a guardhouse at four in the morning under suspicion of murder. I'M SORRY (not sorry).