Why Edward Norton loves the dark side

Here are a couple of things you should know about Edward Norton. Firstly, don't call him Ed. That's not his name, even if you'd like it to be. The second is, try to not be a fool. The 42-year-old Norton isn't one. Reflective and thoughtful in real life, he brings those qualities to most of his roles. Even his Incredible Hulk back in 2008 was a thinker before getting angry and turning green.

He still looks serious even as he offers up chocolate and jokes about the British weather. Perhaps he can't help it. So just how seriously will he fare as the evil mastermind of a big studio franchise? Because Norton has joined the Bourne behemoth. In the fourth film of the billion-dollar series, The Bourne Legacy, he plays Colonel Eric Byer, the architect of numerous secretive defence programmes, including Jason Bourne's. With no Bourne left to chase ( Matt Damon quit at the end of the third film, The Bourne Ultimatum) Byer will have to go after another agent, played by Jeremy Renner.

" I'm not sure my guy is the bad guy," Norton says, frowning as we sit in a bare room – exactly the kind where the CIA would beat up Jason Bourne. " He thinks there is a practical necessity to his actions which fulfill a nobler ambition."

A politician, in other words. It's quickly apparent why The Bourne Legacy writer and director Tony Gilroy wanted Norton for the part. Not every actor has the intellectual capacity for the spy thrillers and not every actor can articulate its moral complexity so well. Black and white? Bourne characters operate in 50 shades of grey.

" If you look at the other movies that Tony Gilroy has made – Duplicity and Michael Clayton – as well as all the Bourne films he has written, I think it's clear that the villain in all his films is the corporate world. There is this cabbalistic relationship between the government and its contractors, whether they are in the defence industry or in pharmaceuticals.

"And there are no heroes in this movie. All the characters are willing participants in the darkness. So there's no one here gleefully trying to take over the world. It's more like they're all enmeshed in a matrix of compromised morality."

"Enmeshed in a matrix of compromised morality." A phrase for our times perhaps? This animates him. "Yes," Norton says. "There is a perception that class warfare is starting to be staged. But what is really interesting to me is that the corporate side has been waging a culture war on us for the last 40 years and now they're screaming at us: 'Don't villainise the rich!' They're crying foul and I just don't think they should. The rich aren't overburdened."

It's fascinating, the use of the word "us".' Norton is the son of a Maryland lawyer and his grandfather was the director of a construction company. When he was younger, he did some consulting work in Japan for the firm. It would have been easy for this Yale graduate to turn to the corporate world himself. What made him decide to run far away from it? He shrugs. "I have to be challenged. I like things that are unfamiliar, worlds that I haven't knocked around in before. And I like to align myself with whatever is good that is going on."

Norton, one suspects, is far more leftfield than most sons of East Coast affluence. After seven years of dating the Superbad and Pineapple Express producer Shauna Robertson, he's now engaged to her. But back in the 1990s, as a young Hollywood buck, he went out with Courtney Love for four years. No matter how much the mind tries to match those two together, it can't.

He has been leftfield in his acting choices, too, despite having two Oscar nominations by the age of 30 – for his 1996 film debut in Primal Fear with Richard Gere and then American History X, where he played a reformed neo-Nazi. He starred opposite Brad Pitt in the 1999 cult classic Fight Club. Yet the Noughties didn't deliver for Norton. The 25th Hour, Kingdom of Heaven, The Illusionist and even The Incredible Hulk weren't good enough vehicles for his talent.

Fortunately, this year he has The Bourne Legacy out, plus Moonrise Kingdom, a Wes Anderson whimsy set around scouting in the 1960s and a sleeper, word-of-mouth summer hit. Although perhaps he just has too many other interests to take on more roles.

An uncredited writer on projects like Salma Hayek's Frida (less controversially, he dated her, too) and The Incredible Hulk, he is also a UN goodwill ambassador for biodiversity. He has campaigned for Barack Obama, saying: "I'm not disillusioned with the last four years, I think we've achieved a lot."

He declares that he "can't imagine what life would be" if he were too famous to take the New York subway, yet had he reprised his role as the Hulk in The Avengers Assemble, which has surpassed The Dark Knight in billion-dollar profits, he undoubtedly would have been. He was furious in 2010 when Marvel broke the news publicly that Mark Ruffalo would replace him, telling this newspaper that the studio had been "unprofessional and dishonest" in their dealings with him.

"The truth about making an action film is that it can be fun, but it can also be really tiresome," he says.

"It might make a very exciting scene as a whole but the doing of it can be very technical and fragmented as a process. I see it as more of an endurance challenge rather than anything to do with acting.

"But I like what Tony has done. He's created a dark, dark world for me to explore here."

A dark world. For all the speculation that an actor like George Clooney will eventually go into politics, perhaps Norton would relish it more. Just don't expect –even then – to be able to call him Ed.

'The Bourne Legacy' is released on 10 August