AFAIK, Charisma only affects the vast majority of conversations through the mechanism of conversation skills. The better the Cha, the better the base score for your conversation skills, but the level of the skill and your dialogue choices are what determine the dialogue options.
I don't think there are any dialogue options that depend on the raw score of a stat (unlike in the 2nd ed-based IWD1 and the BG series), but I could be wrong.
