Tools of Characterization
Characterization in The Dark Knight
Vocalization
People make a lot of fun of Christian Bale's gravel-tastic voice as Batman, but it's more than just sounding tough. It's part of his disguise, and for a guy like him, that's really important. He's Bruce Wayne! Everyone knows who he is, and everyone's seen him talking on the TV. If he gives any hint of his real voice, the game is up. Nolan, being all about a believable Bat, wants us to buy into the fact that he's a mystery. Mock the gravel all you like (and believe us, plenty have) but poor old Bruce doesn't have any other options if he wants to avoid serious jail time.
Similarly, the Joker has a kind of a high-pitched squeaky voice, something you'd expect a clown to have. It's kind of disarming, until you hear the rumbling roar underneath it and it suddenly turns into the stuff of nightmares. "Look at me!" he snarls to the poor Bat-wanna-be he has in his clutches, and you can hear the maggots rotting in his soul. Some jokes just aren't that funny… and if you listen, it doesn't sound all that different than Batman's. Hmmmmm…
Harvey Dent plays the same trick, though on a much more subtle level. He's a testy boy, our Harvey, and when he gets really angry, you can hear his voice drop into a deep, menacing growl. Nolan uses it as foreshadowing while Mr. Dent is still in Upstanding Citizen mode: letting us know that he's got some serious anger issues that might just drive him around the bend. Dialogue informs character, but in this case, it doesn't hurt to occasionally sound like you have a burgeoning case of throat cancer.
The Joker's Make-Up
As comic book fans known, the Joker's white face is stuck that way. Whatever happened to him that turned him into the Joker, it was permanent, and if he wanted to look like a normal person, he needed to put on make-up. (You can see this in action in the Tim Burton version of Batman.) Nolan wanted to stick with fastidious reality, however, so the Joker's perma-pale became ordinary make-up that the character always wears. (Pity we can't say the same about the scars beneath it.)
But there's more to it than maintaining our suspension of disbelief. The Joker had some things to say, after all, and his make-up was a great way to say it. He believed that everyone was a monster underneath, that if he pushed them far enough, there wouldn't be any difference between him and them, and he thought that everyone was constantly lying to themselves about it. Everyone thought they were better, more noble, more human than he was: it became masks that they hid behind, at least as far as he saw it.
He didn't want to play by those rules. He didn't want to hide who he was; he wanted everyone to see the monster that they denied. So he painted himself up in the scariest clown make-up of all time and started leering at the passersby. "This is who you are underneath," it tells us. "I'm just the only one who's honest about it."
Heck of a lot scarier than a death ray or an army of robots, isn't it?
Two-Face's, um, Face
Not hard to see the symbolism here. Harvey Dent's always had two sides to his personality, which is what earned him the name "Two-Face" when he was with Internal Affairs. There's the nice guy, who charms the socks off his girl and takes big risks to bring justice to Gotham. And then there's the angry guy: the one who knocks a hostile witness cold and lets the Joker's crazy underling think he's going to be murdered just so Dent can get some answers.
That darkness is always underneath his shining façade: tugging at the corners and inducing him to do some really, really terrible things. In fact, the Joker's whole darn point is that everybody has that special kind of awful in them, and if he can bring it out in the people's chosen champion, then no one's safe from his poisonous philosophy.
Two-Face's face just brings all of that up to the surface: showing us the divided side of his soul and making sure we see how horrible and ugly it makes him when he gives in to the dark side. In some ways, it's even scarier than the Joker. After all, Harvey doesn't start out evil; we just see him get there step by agonizing step.
Costumes
Bruce Wayne wants to be a menacing creature of the night, striking fear into the hearts of criminals everywhere. Hence he wears a black cape and cowl, as well as some cool black body armor to keep those bullets and knives from puncturing anything permanent.
The Joker, on the other hand, dresses in purple and green suits. a color combination that any artist will tell you is designed to cause the heebie-jeebies. It's custom-made, too, which emphasizes another characteristic of its own: his anonymity. They can't trace him through his tailored threads, so they have no idea where he came from or who he was before he decided to bring Gotham crashing to the ground.
Then there's Harvey, who looks quite the GQ cover model as a crusading district attorney: nice suits, power ties and shoes of imported leather. That changes like his face after the accident, with one side staying neatly pressed and the other side looking like someone poured melted strawberry shakes all over it.
In all three cases—and with most of the supporting cast, who wear practical workaday clothes that reflect their jobs—the clothes really do make the man. If we want to see who they are, we can just check out what they're wearing. It doesn't tell us everything, but it gives us a pretty nice starting point to get going.
Ideals
Philosophy plays a surprisingly large role in The Dark Knight, and the characters' conflicts usually boil down to the things they believe in (or in the case of the bad guys, what they don't believe in). Batman, for instance, wants the people of Gotham to have hope again: to know that they don't have to surrender to the punks and the criminals, and to fight to take their city back from those who would destroy it.
The Joker, on the other hand, believes the exact opposite: that deep down inside everyone is as awful as he is and that if he only pushes hard enough, he can turn everyone in the city into a monster. He's got a problem with rules too. He sees us breaking the very laws we tell ourselves we can't, even if it's just a little thing like Harvey pulling strings to get a table at an expensive restaurant. He loathes the perceived hypocrisy. Batman counters him with the one rule he'll never break (no killing), and the last third of the movie is basically an extended test to see whose philosophy will crack first.
Poor old Harvey gets caught in the middle. He starts out like Batman, and in fact he's even more pure than Batman, since he's not resorting to vigilante justice to save the city. His ideals start to crumble as the Joker puts the whammy on him; they fall apart completely once Rachel gets killed, and his face ends up looking like something scraped off the oven at Pizza Hut. There, too, his philosophy shifts: going from a straight law-and-order man to a deranged sociopath whose every decision is determined by the flip of a coin. We can see the descent in his shifting outlook, and as with the other two main characters, in the way his thinking governs his actions.