The other day a friend mentioned that he thought his girlfriend was probably a narcissist. As an example of her behavior he mentioned that she would constantly interrupt him, but get angry if he ever interrupted her.
Those who get most enraged when interrupted do seem to do the most interrupting themselves. (In both cases, they must dominate the conversation, a sure sign of narcissism.)
The other behavior my friend cited was that if he so much as talked with other women, she would get angry, and say it was inappropriate, and hurtful to her. At the same time she would brazenly flirt with other men, and once even received flowers from another guy. She would then dismiss my friend's objections by saying that these men were just friends and he was being silly.
Hypocrisy may be the best measure of narcissism there is. There's no one more easily annoyed than a hypocrite, and there's no one more annoying.
Another measure is how much the person wants to talk about himself vs. how willing is he to listen to others talk about themselves. There's a guy who swims at my pool who likes to talk about himself, but if anyone else dares to do the same, he'll just say, "I have to loosen down now," and put his head underwater and push off the wall. (I'm not the only person who's noticed this.)
This guy isn't evil; he's not a sociopath. He's just a hypocrite (and demonstrates this in other areas as well). Like most such, does not see his own hypocrisy. (I actually told him once he was a narcissistic personality, which he didn't appreciate.)
The other day I swam a set with him and another guy. I was complimenting the other guy on his swimming, and the narcissist seemed to get increasingly annoyed. I eventually realized that he was annoyed that I was complimenting the other fellow rather than him.
A narcissist has to be the center of attention.
Your garden variety narcissist is not a sociopath: you don't have to dive for cover when you see him. But hypocrites are never good company.