I'm not there yet. But I'm working on it. Especially in a trilogy set in ancient times, this turns out to be hard, because the things that are automatic clues to character 3000 years ago probably mean nothing to us, and vice versa. Consider:
- "Hey, man, like, don't oppress me, y'know? Like, just get along, groove with the rhythm of the universe. Come down off that square you're riding, man, like, seriously."
- (On cell phone in restaurant, loudly): "You're killing me, man. No, you listen. I'm trying to do you a favour and you can't get your *** together. Come up with five big ones by 4pm today. I know a half a dozen guys who'd kill to get in on this IPO. So hit me with the cash or you'll spend your the rest of your pathetic life wishing you had."
So what's an author to do? Fall back on the universal constants whose behaviour is probably the same in any era. The person who leans into your face and shouts is probably always going to be a bully. Or a drill sergeant. The girl who bats her eyelashes at a boy and asks him to help her move something "because he's so big and strong" is probably a flirt in any era. Unfortunately these are also the rankest of stereotypes, and they tell us nothing about the person's social standing or profession, but at least they're a starting point.