Director was "nervous" mounting Batman film sequel

"There are very, very few good sequels, I think," he said. "The two I always had in mind, that we were aspiring to be, were "The Godfather II" and "The Empire Strikes Back," the 1980 follow-up to the first, smash hit "Star Wars" movie.

"Other than those, (good sequels) are pretty thin on the ground," he added.

Fortunately for Nolan and the Warner Bros. movie studio, it appears he may have landed on terra firma with "Dark Knight," which has earned strong early reviews in part due to the performance of Heath Ledger as Batman’s nemesis, the Joker.

Nolan said the trick to making a good sequel is staying true to the tone of the original film, but at the same time offering audiences something fresh and exciting.

"For instance, right from the start, I wanted this growing feeling about Batman -- that what he’s doing isn’t working and it’s being misinterpreted by the people of Gotham," he said. "I also wanted to make it more expansive and just push it further in all directions.

NO JOKE, THIS JOKER

In "Batman Begins," Nolan successfully restarted the "Batman" franchise by giving the movie a dark and ominous tone, and hiring Christian Bale to play a man conflicted about his life as playboy millionaire Bruce Wayne, who eventually becomes crime-fighting superhero Batman.

The first film deals with Batman establishing himself as an extraordinary presence in his home town of Gotham City. "Dark Knight" has him dealing with the criminals’ response.

The anarchic Joker intimidates politicians, the police, the public and even the Mob, as he carefully orchestrates a series of murders, kidnappings and bombings to bring Gotham to its knees. Only Batman seems capable of halting his rampage.

"I always felt that Batman was based on the idea of taking very negative, dark motivations and energy, and trying to do something positive with it," Nolan said. "He’s clearly wrestling with all that. And you need a great actor to be able to express all that, and Christian is so good as Batman."

Bale returns as does Michael Caine as Alfred the butler, but Nolan shook up other key roles. Katie Holmes is out as Rachel Dawes (Wayne’s love interest), replaced by Maggie Gyllenhaal. Aaron Eckhart arrives as a new District Attorney.

The boldest step was casting Ledger, perhaps best known for his Oscar-nominated role as an introverted gay cowboy in "Brokeback Mountain," as the flamboyant and manic Joker.

Nolan said Ledger, who died this year of an accidental drug overdose, plays the Joker as "someone completely devoted to chaos," yet he is still "human enough to be frightening because if he’s not human, you won’t believe in him."

His initial concerns seem to be have been unfounded, since most "Dark Knight" reviews have been positive. Richard Corliss of Time Magazine, for example, said it is "bound to haunt you long after you’ve told yourself, ’Aah, it’s only a comic book movie.’"

(Editing by Bob Tourtellotte and Eric Walsh)