....sitting on a chair, with my feet propped up on another chair, half-intending to occupy both until my co-workers arrive, because this is the kind of party where chairs are precious, territory-establishing objects. But I'm not thinking that I'll fight for the feet-propping chair if someone else asks for it, I just want to casually comandeer it for as long as possible. Up comes this mophead hipster-looking white guy in a denim jacket. I think he may ask for the chair. What he does is makes a strange little whooping noise, point at something on the ground as if to distract my attention, and then grab the foot-propping chair away, again with a little nonsense noise like "Shwoo!" He takes the chair to another corner of the party, sets it down, and starts gabbing with his friends.
...now, I'm not one to make snap judgements about people based on their appearance, but this guy was obviously an asshole. It's one thing to be Too Cool to ask for a chair, instead making a big show of stealing it with your clever "what's-that-thing-on-the-ground" trick. It's another thing to do all that while sporting an obnoxious lead-singer-of-the-Strokes hairdo and making little comic book noises. I'd give him the benefit of the doubt if he was on drugs, but then again if that was the case, I'd figure him to have on some appropriately ironic T-shirt that said something like "I AM ON DRUGS" in pink block letters.
...I have another theory of this guy that is based completely on my brief impression. He probably doesn't know enough assholes in his personal life. There are certain people in LA who feel it's their job to act out the asshole at every social function, because they're a cool-rebel-rocker-actor-poker-player-whatever. They live in sheltered shmoozy environments where everyone's superficially civil to each other, or else dulled and jaded by the sterile isolation of modern life, etc. So they "shake things up" by acting in the style of jerks that they've seen on TV. They don't actually know any real assholes, but they are repelled by the fake niceness that permeates LA, and feel this is their way of Keeping It Real.
....This is why it's important to know some true assholes in your life. So you don't waste time posing as one to hide your own lack of character traits. Again, I could give mophead benefit of the doubt and believe that he was a genuine-article asshole who came from a whole family of abusive chair-stealing types. But I don't think so. He didn't seem to have the real sociopathy in him, merely a surface caricature of it. Like the kind of guy who responds to someone saying "Hi!" by lunging at them and screaming "Krakoa! Kraoka!" just to point out how meaningless the convention of saying Hello is. I'll assume that he's the bad boy in a pack of nice friends, and his friends all think he's hilarious.