LOS ANGELES — Three weeks before shooting began on "The Dark Knight," Heath Ledger called director Christopher Nolan to discuss an epiphany he had about his character, the Joker.

The key to the demented killer, Ledger said, was his voice. Without a menacing hiss, it would be a retread of Jack Nicholson's satirical turn when he played the character in 1989.

"He said he found the perfect one," Nolan recalls. "He had been studying ventriloquist dummies and thought they had terrifying voices. He was going to emulate one. I remember saying, 'Yeah, that sounds great.' When I hung up the phone I thought, 'God, I hope that's going to work.'"

There's little question now. The demonic voice floats through the myriad trailers promoting the film, opening Thursday night in many cities. And the late actor's performance — eight months from the Academy Awards — is garnering Oscar talk.

"He wouldn't even really rehearse with the voice," says Christian Bale, who plays Batman. "He held it back a little, waiting for the cameras to roll. But when they did, we knew he was on to something special."

Great villain voices usually are, says film author and historian Leonard Maltin.

"It can be the most memorable element of the character," Maltin says. "You think about the great ones: Anthony Hopkins' Hannibal Lecter or the great Peter Lorre or the granddaddy of them all, Darth Vader. In a way, those characters are defined by that voice."

But what makes for an effective one? It's more than theatrics, more than a growl or a snarl.

Some directors prefer bad guys who cackle. Others like villains whose pitch never changes, even when they do terrible things.

"It's tough to define, but you know it when you've found it," says Sam Raimi, director of the "Spider-Man" franchise, who has cast Willem Dafoe, Alfred Molina and Thomas Haden Church in evil parts.

"Personally, I love a villain who has a good laugh," Raimi says. "My favorites are Margaret Hamilton in 'The Wizard of Oz' and the Nazi in 'Raiders of the Lost Ark.' Those people who can laugh when they're committing atrocious acts are truly horrifying."

Heavies without good voices, says Steve Mason, a columnist with Hollywood.com, are a little like guns without bullets: frightening on the surface but harmless when it's time for action.