Christopher Nolan has notched up an impressive body of work in just 10 years, including such critically acclaimed pictures as Memento, The Prestige and Batman Begins. The director talks to David Grove about The Dark Knight – a movie many critics are already calling the best Batman movie to date.

Why did you want to put both The Joker and Two-Face in the same movie?

I feel that when you make a sequel to a film like Batman Begins, everything should be bigger, the scope of the film should be bigger, the world should expand. We introduced The Joker at the end of Batman Begins and he’s an icon, almost on the level of Batman. With Two-Face, we firstly introduce Harvey Dent, played by Aaron Eckhart, who’s this charming and decent District Attorney who teams with Batman and Gordon to fight The Joker. Then something happens to Harvey Dent and everything changes and there’s a lot of complications.

I just felt having these two villains would make for a very exciting and chaotic mix. This isn’t just Batman’s movie, it’s The Joker’s movie and it’s also Harvey Dent’s movie.

How would you describe the relationship between Batman and The Joker in The Dark Knight as compared to Tim Burton’s version?

We treat The Joker in this film as an icon, just like Batman is an icon. In terms of the origin of The Joker, my brother Jonathan and I decided to go right back to the beginning of the character, to the first few Batman comics, for inspiration. He’s a terrifying character in this film, a spectacle, and he’s not just some petty criminal who’s looking to make a score or make easy money as he was in the 1989 film. He’s a murderous character, a sick monster, and I would describe The Joker as being an absolute character in terms of his murderous nature. This is a sinister Joker.

What was it about Heath Ledger that made you feel he was the right actor to play The Joker, and could fill Jack Nicholson’s shoes?

The fact that I cast Heath tells people that we’re going in another direction with The Joker, so I wasn’t even thinking about comparisons to Jack Nicholson. I was looking for an actor who wasn’t afraid to take risks, to go to the dark places, to try anything, and Heath came to mind. We’d crossed paths before in terms of meeting about other projects and we very much wanted to work together. He’s an energetic and intense actor and when we talked about The Joker character I knew he was the right actor for the role. I actually picked Heath for The Joker before we had a script.

The Joker in this film seems to be more rooted in reality than previous incarnations…

When I made Batman Begins, my main goal was to bring realism to the film. Everything - from the creation of Batman’s costume, to the Batmobile - had to be feasible and realistic. I brought that same approach to this film. The Joker is a very realistic character in that all of his actions come from a place of psychological reality. His face isn’t white because he was sprayed with chemicals but because he puts make-up on, and his face is horribly scarred from a knife attack and his disfigurement drove him further over the edge. The scarring also explains the sinister smirk on his face, so everything about him is organic. We gave The Joker a punk sensibility in this film, both in terms of his look and personality. He brings chaos and anarchy to Gotham City. He’s the ultimate anarchist.

You also explore Two-Face’s origin story.

Yes, we see Harvey Dent as this District Attorney who’s determined to bring criminals to justice. He’s an honest man and he forms an alliance with Gordon to clean up Gotham. When he transforms into Two-Face, it’s very shocking because he goes from being this good and decent man to being a killer. Harvey Dent goes through the biggest change in the film as he changes into Two-Face. We show the whole transformation from one extreme to the other.

Why did you and your brother, screenwriter Jonathan Nolan, decide to return to The Joker’s first appearances instead of the later versions of the character?

The great thing about making a Batman film is that you have over 60 years of materials to work from, lots of great ideas and concepts to draw from. I just found that the early versions of the character had the elements I was looking in terms of him being a practical villain and a villain who has a destructive and murderous personality. The Joker committed a lot of murders in his early appearances as compared to the later versions where he was just a criminal and joked around a lot more.

How has Batman changed since the previous movie?

He's much more comfortable in his own skin now, in the role of Batman. In Batman Begins, it was a whole hour before we saw Bruce Wayne as Batman because that was Bruce Wayne’s story, and I wanted to totally flesh out the character and take my time in showing his transformation into Batman. Now he’s Batman, and his feelings about his parents and childhood are all buried away.

The biggest difference in this film is that we emphasise the detective part of his personality. Besides being a crime-fighting vigilante, Batman has always been a brilliant detective in the comic books and we show that in this film. I’ve always thought of Batman as being a great detective and I wanted to show him solving mysteries and using deduction instead of just violence. I’ve always thought that Batman’s greatest weapon was his brain.

Are you interested in directing more Batman films?

I can’t say never. But there’s other film projects that I very much want to do and making a Batman film is such a big commitment that I don’t know if I want to do that again. I felt the same way after Batman Begins, so you never know. I approach every film like it’s the last film I’m ever going to be allowed to make, so I don’t ever think ahead. At this moment, I have no ideas or thoughts of what a third Batman film would be about, or what villains could be used, nothing.

The Dark Knight is out now in US cinemas and opens in UK cinemas on 24 July 2008.