It's not the usual kind of charisma—I didn't feel a strong need to win his approval, relative to how much I do with other smart people. It's rather that he was extremely quick to understand my arguments and point out important aspects I hadn't considered, which makes it easier for me to consider that my argument might be flawed. So that's an aptitude, but it's one better correlated with good argument than the aptitude of charisma is.