A record number of moviegoers packed theaters to see the new Batman blockbuster "The Dark Knight" over the weekend -- and chances are pretty good that they can't shake loose the haunting laugh from late star Heath Ledger's The Joker.

In a recent @ The Movies interview, director Christopher Nolan said that the laugh was entirely Ledger's invention.

"When I was at work on the script and he was trying to figure out how to create this character, he called me to talk about a number of things, including how he was working on a laugh," Nolan said. "He wouldn't do it for me, though, he said he was just working on it."

Nolan said that it was a relief that he didn't have to guide Ledger in coming up with a cackle for one of pop culture's most iconic characters.

"I think one of the hardest things for an actor is to force a laugh. The Joker has to have a great laugh, and he figured out this wonderful way of doing it," Nolan said of the actor, who died in January at age 28. "It almost sound half-hearted to me the first time I heard it, like he was holding something back."

But then, Nolan said, the full impact of Ledger's creation hit him once he saw it on film.

"Then I realized that it has this incredible kind of decay in it some it, somehow," Nolan said. "It's a mocking laugh and a creepy laugh, and I think part of the creepiness is this decay is in everything, including his makeup and the way he moves and everything. There's a sense of corruption in it."

Starring Christian Bale in the title role, "The Dark Knight," which chronicles The Joker's rise as a crime figure in Gotham City, shattered box office records throughout the weekend -- including a $66.4 million take on its opening day and more than $155 over the weekend.

The sequel to the 2005 blockbuster "Batman Begins," "The Dark Knight" also stars Michael Caine, Gary Oldman, Aaron Eckhart, Maggie Gyllenhaal and Morgan Freeman.