If there's one thing I like about my superhero movies it's when they have a theme. When they have a theme AND their own style. A theme, their own style and are true to the essence if not the facts of the comic. Well, a theme, their own style, the essence of the comic and an almost fanatical devotion to the pope. Wait. Let me start again.
A theme. Right. I love superhero movies to have a theme, an added layer that makes it more than a by-the-numbers revenge story (which they often are). It's what 1989's Batman didn't really have. It's what screwed up Spider-Man 3 (Venom not being part of the theme). But The Dark Knight? Its central theme is captivating.
There be spoilers ahead. Turn back, ye damned souls who haven't seen the movie yet.
Essentially, the theme is people making moral decisions between the lesser (they hope!) or two evils. How do you choose between the rock and the hard place? What does a moral person do when confronted by two immoral choices? It's Sophie's Choice with masks. And the lynchpin is the Joker's modus operandi.
By making "the difficult choice" (no, strike that, "the IMPOSSIBLE choice") his crime du jour, it highlights every non-Joker-related choice in the film.
The Joker's Gambits
Look at the clown prince's acts of terrorism. He kidnaps Rachel and Harvey, warns the good guys, but only one can be saved. Batman chooses Rachel, but doesn't count on the Joker flipping the addresses. For the Joker, it's about chaos, about showing how choices are essentially random acts and how the details of your life are out of your control. Sophie's choice subverted.
The hospital dilemma: The Joker turns Gotham's population into potential murderers when he puts a price on Reese's head. That price: If you don't, he'll blow up a random hospital, maybe one where you have a loved one. It's also a sequence that gives us Nurse Joker, a wonderful scene that may well make you laugh at an act of terrorism. Talk about making the audience walk the moral gray zone.
The ferry dilemma: Another great set piece. Two boats, one filled with convicts, one with ordinary citizens. Each is given a detonator with which to blow up the other boat before midnight when both boats blow. Who pushes the button first? Would you rather die or murder? It's an amazing part of the film that generates a lot of tension despite the fact that we don't know anyone aboard either ferry! To me, the best reaction is the inmate who throws the detonator out the window. How do you resolve an impossible choice? Throw that choice away. It wasn't yours to make in the first place.
The Joker gives impossible ultimatum after impossible ultimatum in TDK. You can capture him, but it means the police station will blow up. You can shoot his clowns, but they'll turn out to be hostages. You can unmask publicly or the mayor can be killed. Your choice. His greatest achievement (and triumph) however is...
Here is a character that embodies the random moral choice, forged in the Joker's wave of terror. Harvey does what he thinks is right, going after the mobs with gusto and no regard for the danger he places himself in. The same goes for his taking the fall for Batman, attracting the Joker's interest. Result: Despite his good intentions, he loses everything, including his sanity. From there, he is a conceptual instrument of the Joker, also giving people impossible choices, culminating in Gordon's own Sophie's Choice - which of his family does he love the most?
In the end Batman takes the fall for HIM, so that Gotham doesn't lose its "white knight". It's a big sacrifice for both him and Gordon to make, one that can serve to drive the next movie. One might say, however, that Harvey Dent has been a polarizing figure since the beginning. He is Rachel's AND Gotham's alternative to Bruce/Batman. Will she choose security or the man she really loves? Will the city embrace its White Knight or its Dark Knight? Before his accident, Harvey is a proponent of the non-choice, the coin a two-headed deception. An absolute decision with no alternatives. That certainty is what he loses. The Joker wins that argument.
The mobs: To keep getting hit by the Joker or to come together as a cartel whose funds are all handled by the same man? It's a choice that leads to their mass indictments.
The population: To endorse (and even become) vigilantes to rid the streets of crime, or to place your faith in the system, which fails to protect its public servants?
Alfred: Deliver Rachel's final message or allow Bruce to believe a lie?
Lucius: Can he compromise his values pertaining to Batman's version of the Patriot Act if the threat level is high enough?
Mr. Reese: To reveal or not Batman's identity. Is it worth the risk? What if a crazed lunatic has promised to continue murdering people unless that identity is revealed? (Another moral choice the Joker flips.)
Ramirez: Oath as a police officer or mom's hospital bills paid by the mob?
Gordon: Allows his family to think him dead for the sake of a sting operation that captures the Joker. Again, the Joker flips that "right" action and turns it into the absolute wrong one.
Bruce Wayne: As the hero, he's confronted by many choices, but central to his arc is his decision to quit being Batman. Would that really be the best thing to do? And yet, without a Batman, it's unlikely we would have a Joker. Also note how that decision is usurped by Harvey Dent. Why doesn't he come forward then? Hard questions.
In The Dark Knight, you're perpetually damned if you do, and damned if you don't.