How we cite our quotes: All quotations are from The Dark Knight.
Quote #1
BANK MANAGER: Criminals in this town used to believe in things. Honor, respect. Look at you! What do you believe in? WHAT DO YOU BELIEVE IN?!
THE JOKER: I believe whatever doesn't kill you simply makes you… stranger.
We're setting the battle lines early on. Right away, the film sets the Joker apart from more ordinary criminals who clearly don't get his special little vibe. The rules are changing in Gotham, and as Batman puts the normal gangsters away, the freak show is rising to fill in the gap. On a deeper note, it gets back to the idea that the Joker is the Devil, punishing the wicked (like this bank manager) as much as he drives the righteous around the bend. Principles get put to the test in such an environment—including the Mafia's.
Quote #2
HARVEY DENT: I make my own luck.
Harvey's jabbing the system here. He's willing to break the rules to make things come out the way he wants them. Already, he's showing us that the system isn't as pure and good as it should be, and that maybe, just maybe, the Joker has a point. He makes a similar compromise a few scenes later: pulling strings at a high-end restaurant to take Rachel on a date.
Quote #3
LT. GORDON: I don't get political points for being an idealist. I have to do the best I can with what I have.
Like Dent, Gordon is a man of principle. And if you saw Batman Begins, you know how tough that was for him. But he has to compromise, too. The difference? He acknowledges it, unlike Harvey, who refuses to bend until he breaks. That may be why Gordon isn't all scabby and oozing (or indeed, threatening to murder children) by the end of it.