Director talks about Ledger's final film - Batman

SYDNEY - Heath Ledger had a unique charisma that affected everyone on the set of his final film, The Dark Knight director Christopher Nolan says in a written tribute to the Australian actor.

Nolan, the director of the yet-to-be released Batman movie The Dark Knight, has told of working with Ledger, describing him as a thoughtful and creative actor.

Ledger plays a psychotic, mass murdering, skateboarding Joker in the film, which will be released in July and is already received rave reviews.

Nolan said Ledger's charisma was so catching that younger people working on the set would turn up on skateboards.

"I'd fretted about the reaction of Batman fans to a skateboarding Joker, but the actual result was a proliferation of skateboards among the younger crew members," Nolan told US magazine Newsweek.

"If you'd asked those kids why they had chosen to bring their boards to work, they would have answered honestly that they didn't know.

"That's real charisma invisible and natural as gravity. That's what Heath had."

Nolan also told of Ledger's thoughtfulness, saying that after asking everyone on the film set to stay back one night, he quietly thanked each crew member.

"Everyone seemed to understand that Heath had something special and that we had to capture it before it disappeared," Nolan said.

He described two short films that Heath had made as "exciting and haunting".

"I've never felt as old as I did watching Heath explore his talents.

"Heath was bursting with creativity."

Nolan said he had been looking forward to the day when he'd show Ledger the finished film.

"Now that screening will never be real," he said.

"I see him every day in my edit suite.

"I study his face, his voice. And I miss him terribly."