[SixOfSpades]

Some conversations check the PC's stats, some check those of the "party leader" (whoever's portrait is at the top of the right-hand column), and some check those of the person who's actually doing the talking.

Many conversations check for CHA, but the Ring of Human Influence is used so commonly that the low-CHA conversation options for many dialogues are never even seen.

Only Neutral or Evil speakers are given the option to betray Garren Windspear.

It's the CHA of the party leader that matters when buying things in stores.

The best example of stats determining conversation options in the whole game is during the Mae'Var quests, when Edwin tells you to get some documents off a fat guy named Marcus. Now, anyone can simply buy the letters off Marcus, and if you're a Rogue you can pickpocket them (with varying odds of success), but if your CHA is high enough, you can intimidate Marcus into simply giving you the goods, and if your STR is high enough, you can (break his neck quickly and quietly).