How we cite our quotes: All quotations are from The Dark Knight.
Quote #1
BANK ROBBER 1: So why do they call him the Joker?
BANK ROBBER 2: I heard he wears make-up.
BANK ROBBER 1: Make-up?
BANK ROBBER 2: Yeah, to scare people. You know, war paint.
In this case, the most important thing about the Joker, the only thing that matters, is what he looks like. We know everything we need to know about this guy just by looking at his face.
Quote #2
NATASHA: What if District Attorney Harvey Dent is really the Caped Crusader?
HARVEY DENT: If I were going out at night, someone would notice.
Harvey's basically saying that, in order for Batman to be Batman, he can't have a normal relationship with anyone out there. He's probably right, which is why it takes his fiancée dying—cutting off his relationship with the person he cares for the most—to fully turn him into Two-Face.
Quote #3
GORDON: What's he hiding under there?
He asks this question about the Joker, but not about Batman, which is interesting. Batman started out breaking the law, just like the Joker did, and yet Gordon saw something there that separated him from other criminals. Seriously, the Joker's ripping off mob banks when Gordon says this! Maybe the identities of hero and villain aren't as separate as we think…
Quote #4
THE JOKER: Don't talk like one of them. You're not! Even if you'd like to be. To them, you're just a freak, like me.
We see the Joker identifying with Batman a number of times in this film. In fact, he seems kind of peeved that Batman isn't standing with him about the squares. Are these two really closer to each other than the rest of Gotham? As scary as it sounds, the answer is probably "yes."
Quote #5
ALFRED: Will you be wanting the Batpod, sir?
BRUCE: In the middle of the day, Alfred? Not very subtle.
ALFRED: The Lamborghini, then. Much more subtle.
Once again, Alfred cuts to the heart of it. Bruce will always be Batman, regardless of what he's driving and when he's driving it.
Quote #6
HARVEY DENT: Remember that name you all had for me when I was at Internal Affairs? What was it, Gordon?
GORDON: Harvey, I...
HARVEY DENT: Say it. SAY IT!!!
GORDON: Two-Face. Harvey Two-Face.
TWO-FACE: Why should I hide who I am?
This is Harvey's dark acknowledgement that Two-Face was always a part of him, even before he became DA. We suspect that that hurts him a heck of a lot more than those burns do.
Quote #7
THE JOKER: I had a vision, of a world without Batman. The mob ground out a little profit and the police tried to shut them down, one block at a time. And it was so...boring.
Kinky…The Joker defines himself here not by his own actions, but by the fact that he needs Batman to be completed. His world is no longer worthwhile unless he has someone like Bats to challenge him: probably the closest a guy like him has to a relationship with another human being.
Quote #8
THE JOKER: Tonight you're all gonna be part of a social experiment. Through the magic of diesel fuel and ammonium nitrate, I'm ready right now to blow you all sky high. Anyone attempts to get off their boat, you all die. Each of you has a remote... to blow up the other boat. At midnight, I blow you all up. If, however, one of you presses the button, I'll let that boat live. So, who's it going to be: Harvey Dent's most wanted scumbag collection, or the sweet and innocent civilians? You choose. Oh, and you might want to decide quickly, because the people on the other boat might not be so noble.
This is a bet over who people are: are they essentially awful, as the Joker maintains, or ready to believe in good, as Batman's convinced they are? The whole story boils down to which side those people on the ferry choose, and by extension, who they decide to be.
Quote #9
BATMAN: The Joker cannot win. Gotham needs its true hero.
GORDON: No!
BATMAN: "You either die a hero, or you live long enough to see yourself become the villain." I can do those things, because I'm not a hero, not like Dent. I killed those people. That's what I can be.
GORDON: No, you can't! You're not!
BATMAN: I'm whatever Gotham needs me to be. Call it in.
Batman's identity is flexible, amorphous, and able to be twisted around like Silly Putty if Bruce Wayne wishes it so. In this case, he believes it would do the most good if Batman becomes a villain: taking Harvey's identity onto himself in order to preserve the good man that Harvey used to be.
Quote #10
GORDON: He's the hero Gotham deserves, but not the one it needs right now. So we'll hunt him. Because he can take it. Because he's not our hero. He's a silent guardian. A watchful protector. A Dark Knight.
Gordon cements Batman's true identity here one last time before their agreed-upon lie takes effect and he turns the dogs on The Caped Crusader. It's a way of reminding him who Batman truly is before he gets caught up in denying it.