One of the primary conceits of The Dark Knight is that the Joker needs Batman. Near the final climax, there's a speech about it, surprisingly long considering that the Joker's hanging upside down, several stories up, and outside a shattered window when he delivers it. The Joker's madness—random, anarchic, and undirected—requires order to react against; if Batman weren't so unyielding in his crusade against crime, the Joker wouldn't have anything to throw bombs at.
The Dark Knight—the movie, even more than the character—also needs the Joker. Heath Ledger's riveting, maniacal performance dominates, from start to finish, Christopher Nolan's sequel to the 2005 Batman Begins, saving what might otherwise have been a simple heist picture. Ledger overshadows the too-elaborate initial plot about a Mob money-laundering scheme and makes the rest of the cast look like they're working on an episode of Law & Order. When he's onscreen, you can't look away. When he's not in the frame, you wish he were.
It's an unsettling starring role to watch, and not just because it was the last film Ledger completed before he died in January. (New Yorker reviewer David Denby suggested that it's worth considering whether the demands of this role contributed to Ledger's state of mind at the time of his fatal drug overdose, which is a bunch of condescending claptrap.) In most of his iconic incarnations, from the 1960s Batman TV series and Jack Nicholson's turn in Tim Burton's 1989 Batman to his appearances in the 1980s comics The Dark Knight Returns and Arkham Asylum, the Joker's been capable of some very, very bad things. Ledger's Joker seems capable of almost anything, for any reason or none at all. It's a dark and terrifying potential that Ledger comes close to fully realizing. His immersion in the role makes the Joker's brand of evil physically seductive. His lopsided walk and the way his tongue flaps around the makeup-gobbed edges of his scarred mouth seem built-in.
Of course, every hero needs a villain. But Christian Bale as Batman (and his alter-ego Bruce Wayne) could maybe do with a little less of Ledger's Joker. The tightly wound super-vigilante isn't even the star of his own movie. His dramatic arc, which includes the chance to retire the bat-suit and, maybe, settle down with Rachel Dawes (Maggie Gyllenhaal), is drowned out by the energy of the Joker. Batman's even overwhelmed by Harvey Dent (Aaron Eckhart), the righteous Gotham district attorney who, through a twisted combination of tragedy and malevolence, is transformed into Two-Face. The only principal character less vital to this story is Gotham Police Commissioner Jim Gordon (Gary Oldman), who's absent for the entire middle third of the movie.
Ledger doesn't just rise above the other actors. He's better than the overloaded plot—several plots, actually—should allow. The Dark Knight begins as the Joker, already a fully formed madman with no origin story necessary, appears in Gotham to take over the city's crime syndicate. That storyline ends in literal flames, and the consequences of the Bruce Wayne/Rachel/Harvey Dent love triangle propel a murky transition into the impressive but equally inevitable final showdown.
Ledger's brilliance as the Joker lies in his instinctual, physical inhabitation of the role. As an idea, the Joker's not that interesting. There's only so much drama that utter chaos can sustain. But he's thrilling, much more so than any of the rest of the movie. Watching the Joker sit silently in a police interrogation room beats Batman's first fight scene with a bunch of overweight Batman knock-offs and a gang of pit bulls. Only the truck chase through downtown Gotham two-thirds of the way through injects the rest of the movie with the same kind of white-knuckled thrills that Ledger delivers every time he shows up.
The trouble is that none of the other ideas in The Dark Knight are very interesting, either. Batman's troubled by the ethical dilemmas of vigilantism and his own increasing tendency toward justifying his means by their ends; Dent's fight for justice becomes a quest for vengeance; Gordon spouts a vague, essentially meaningless aphorism about whether Batman is the hero Gotham deserves or needs as the movie's epilogue. It's a shoulder shrug to the ethical questions raised by the very nature of Batman. That's how seriously Nolan takes the ideas in The Dark Knight. It is, however, a great set-up for the next sequel.