It takes a lot to get Christian Bale excited. Surrounded by the ensemble cast of The Dark Knight, he lounges in his chair, rubs his eyes and mumbles through his answers like a latter-day Marlon Brando.

It's clear: he's bushed. And not without reason. In the last year he's knocked off the Batman movie, a new take on the hunt for '30s bank robber John Dillinger for director Michael Mann and is mid-way through shooting a Terminator prequel. He has more right than most to be in need of a siesta.

Then there's the small matter of promoting The Dark Knight. Considered the must-see movie of the summer, it has broken box office records in the US and is on course to be one of the biggest hits of all time.

A lot of the hype surrounding the film has to do with the death of Heath Ledger back in January. To its credit, studio Warner Bros has held back from exploiting that tragic moment and, at the start of our interview, a PR asks that any questions relating to Ledger be confined to his performance as the Joker in the film.

Early noises from the Chicago set of The Dark Knight pointed to 34-year-old Bale's trademark intensity and a willingness to get a little too close to some of the hardware for comfort. A setpiece moment on the edge of the 1,451ft Sears Tower had him near enough to a helicopter to be able to kiss the pilot. Bale smiles at the memory.

"That was probably just me making the story a little bit more interesting than it really was," he admits.

"That was just another incidence of being more experienced in the stunt: jumping off the top of the Sears Tower, standing in the batsuit and looking over Chicago. The helicopter came very close; two inches is a little bit of an exaggeration on my part, but how often am I gonna get a chance to do that again? Probably never.

"Starting on this movie was like going into the eighth month. I didn't just reprise the role since I did seven months on the first one. I was able to do lots more of the stunts myself even though I've got a phenomenal stunt double, Buster Reeves. I'm more than happy to let him jump off five-storey buildings, land on cars, smash into concrete pillars, get burnt and things like that."

A follow-on to the previous adventure, The Dark Knight lifts the lid on Bruce Wayne aka Batman's peculiar brand of emotional psychosis. Perhaps for the first time, the filmmakers have asked the audience to consider whether Wayne's vigilantism has any relevance in a modern society. I ask Bale for his take on the issue. He seems to shake himself awake and, for the first time in the interview, gets animated.

"The thing that's always been interesting about Batman compared to other superheroes is that he has no super powers, unless you include money. He has this altruistic side and then he has this shadow side, which is very violent and bent on revenge. He has to keep them in check, which is why he has this rule that he will not kill. He is very, very tempted to break that rule especially when he comes across an adversary like the Joker who is tempting him to show everybody that at some point everybody's principles mean nothing. These are selfish principles because by killing this man he could be saving many lives. But at that point do his actions become selfish or are they truly good?"

The mention of Ledger opens up the questioning to consider the impact made on the film (and its cast) by the 28-year-old Australian. Everyone wants to know whether he genuinely was affected by playing such a malevolent freak as the anonymous crime kingpin who holds Gotham City to ransom. Bale gives a measured response.

"He did an incredible job with the Joker. Personally, I feel it exceeds any portrayal that's been done before of that character: the anarchic, Clockwork Orange, punk rock approach to it. I enjoyed watching him perform and I enjoyed seeing how much fun he had."

He seems to be at pains to stress his co-star's professionalism and sense of humour, but there is little in his answer that hints at anything akin to a close personal relationship.

And maybe that's the way Warner Bros wants it. Or perhaps Bale really is as exhausted as he appears. It must be wearying, jetting around the world promoting your new film when the ghost of a dead fellow actor looms large over it.