To be an asshole in-game, you gotta be a good guy out of game. If you're playing a pompous greedy noble, let him say "Hmph. I'll let you carry the luggage. Just don't break it." when you find a magic item, instead of hogging it for yourself and saying "But it's what my character would do!" Things like that.

If you're gonna be rushing headfirst into every situation, maybe try half the time instead. Let your friends get their cool heist scene once in a while. After the party inevitably blows their cover and sets the alarm, then you can go ham. You'll always get your chance to shine with a bit of patience.

Basically, you just gotta be sure to let the others get the spotlight too.