My favourite take on Charisma is as follows:
"Charisma: Charisma strictly represents confidence, presence, and force of personality (physical attractiveness is governed by the optional Comeliness attribute; see below). It is therefore analogous to the Willpower attribute from the old Victory Games rules. To reflect this, the Charisma modifier, rather than the Wisdom modifier, applies to Will saves against compulsions, fear, etc. You also apply your Charisma modifier to certain uses of Hero Points (see below). These uses provide a disincentive for everyone other than bards and sorcerers to always make Charisma their lowest attribute.
Social skill is dictated by your bonuses in Bluff and Diplomacy—with your personal confidence and magnetism (Cha) providing a modifier, rather than dictating your baseline. People with low Charisma are typically unsure of themselves, lack presence, and are often ignored. Characters with high charisma scores are heeded; they are leaders, rather than followers.
A character with a Charisma of 1 has insufficient ego to exert executive autonomy; he or she acts as if charmed by everyone he or she interacts with. A character with a Charisma of 0 is dominated, likewise.
How Wisdom and Charisma Interact: A character with a high wisdom (awareness and caution) and low Charisma (confidence and force of personality) is likely to be timid and overly-paranoid about “getting in trouble.” His or her warnings will often be ignored by companions.
Conversely, a character with low Wisdom and high Charisma is likely to be egocentric and careless, assuming that things will “somehow work out.” He can be bold and reckless, like Douglas Fairbanks’ Robin Hood, but he or she will also often need to be rescued by companions, and may, in the worst case, have a tendency to treat others as tools.
A character with high scores in both stats is like Hammett’s Sam Spade―ruthless, domineering, guileful, and always with a backup plan or two."