Talking with Heath Ledger on The Dark Knight's Chicago set, the actor appears exhausted and distracted, putting it down to the fact that he's barely slept. "I got picked up at 4:00am from my house.sorry. I'm completely loopy," he grins apologetically.

If few actors would willingly set about recreating a role made famous by Jack Nicholson, then Ledger likewise expressed his own doubts in taking on Nicholson's iconic Joker. "I certainly had to put on a brave face and believe that I had something up my sleeve. I definitely feared it - but anything that frightens me also excites me," he reasons. "I was a huge fan of Tim Burton's Batman, but there's a big difference between a Chris Nolan film and a Tim Burton film. So there was enough room for a fresh portrayal, and I steered away from what Jack did.hopefully. In preparation, I sat in a hotel room for a month, forming a little diary and experimenting with voices. I ended up landing more in the realm of a psychopath with very little conscience. That's fun to play because there are no boundaries to what he'd say or what he'd do. It's always a very personal process in terms of how you land in the character's shoes. It's a combination of reading all the comic books I could, and also the script, and then just really closing my eyes and meditating on it. Chris and I saw eye to eye on how the character should be played; we had identical images within our minds."

Ledger also found The Joker's garish white make up to be of major assistance. "There's something about the metaphor of working behind the mask and from within a mask," he says. "It gives you the license to do whatever you want. There's nothing consistent about my Joker at all. He's not consistently dark or consistently funny - he's just going up and down the whole time. It's the most fun I've had, and the most freedom that I've ever been given in a role. I'm really surprised that Chris knew that I could do it, or thought that I had something in me like this. It's the bomb."

Ledger pauses for the longest time when I later ask how he was able to embody pure evil. "I don't know," he cautiously replies. "We all have it in us. For a while, I'd be thinking about how to do it, and then I'd just connect to some scary thoughts. It's like eating raw meat - I don't know what that does to your mouth and your eyes. I'd think of simple little visuals like that. That twisted my mind; it feels evil when it's not necessarily an evil thought, but it may look and come across as evil. The rest is trusting your research and trusting all the definitions of the word - psychopath - and then running with that. I haven't seen any dailies, although I could hear them from my trailer. That freaked me out. I actually make a point of not looking at the dailies. It's a waste of time because when I go home, it's too late."

The Dark Knight is released on July 16.