Outside production

A few excerpts from The New York Times:

He is here in London filming the latest episode of the "Batman" franchise, "The Dark Knight." (Mr. Bale, as it happens, plays Batman; Mr. Ledger plays the Joker.) It is a physically and mentally draining role — his Joker is a “psychopathic, mass-murdering, schizophrenic clown with zero empathy” he said cheerfully — and, as often happens when he throws himself into a part, he is not sleeping much.

“Last week I probably slept an average of two hours a night,” he said. “I couldn’t stop thinking. My body was exhausted, and my mind was still going.” One night he took an Ambien, which failed to work. He took a second one and fell into a stupor, only to wake up an hour later, his mind still racing.

He carries his interests around with him, and his kitchen table was awash in objects: a chess set, assorted books, various empty glasses, items of clothing. Here too was his Joker diary, which he began compiling four months before filming began. It is filled with images and thoughts helpful to the Joker back story, like a list of things the Joker would find funny. (AIDS is one of them.) Mr. Ledger seemed almost embarrassed that the book had been spotted, as if he had been caught trying to get extra credit in school.

This is blowing my curiosity-- what are the Nolan brothers filming?