Scorsese's mob rule

His upbringing has informed his entire oeuvre, he says, but most especially the films dealing with the thin line between the right and wrong side of the law.

"I used to listen to my father talk every night about what's right and wrong in the world. My neighbourhood was a microcosm of America . There were hard-working blue-collar people trying to live decent lives but at the same time - and decent people always get upset with me when this comes up, but it's true - there were criminal elements. Because my father wasn't educated he had to learn to live his life not being one of them, but according to the way they said life had to be.

"I was like walking a tightrope for him," he continues passionately. "He was one of nine children and one of his brothers was one of these troublesome figures. Later on, when I made Mean Streets, I thought I was making it about myself and my old friends, but after my father died, I realised I made it about him and his brother." Scorsese was a sickly child and he still suffers from chronic asthma, but his time off school enabled him to develop a love for movies and, in 1964, he entered New York University's film school.

By 1967, he'd made his first feature, Who's That Knocking at My Door? and had become friends with a group of young film-makers who were about to have a profound effect on American cinema - Francis Ford Coppola, George Lucas and Steven Spielberg. Scorsese made his breakthrough in 1973, with Mean Streets, and went on to create landmarks of that decade, including Taxi Driver (1976) and Raging Bull (1980).

The film that started the Seventies new wave of great American cinema was Easy Rider in 1969, which also introduced Jack Nicholson to the world. It's fascinating, almost 40 years later, to see two titans of that period - Nicholson and Scorsese - working together for the first time.

"I wanted a real presence to carry over Francis Costello, so that even if he were only in three scenes, you'd still feel the presence of a character who's almost a god-like figure in the story," says Scorsese.

"I said to Jack, 'Look, I've done this kind of film before. We dealt with major crimeland figures or gangsters, but audiences are going to be expecting something different with this character because of the combination of Nicholson and myself.' So Jack wanted to do a character that was entirely fictional and not based on the character in the script nor based on any real-life crime figure."

The result is explosive. Nicholson has played varying shades of evil before, in The Shining, The Witches of Eastwick and Batman, but in The Departed he is a fully realised, cold-blooded monster whose sick sense of humour is combined with acts of sickening violence. Scorsese even had to temper down Nicholson's role because, he says, "with Jack, a little bit goes a long way".

"There was a scene we shot where Jack winds up with two women in a hallucinatory cocaine fantasy, but we ultimately decided it would be more interesting if that was implied. The relationship between sex and violence in the picture, particularly with Nicholson's character, is obscene, and we pushed that as far as we could, but ultimately some of it fell away in the shaping of the film."

The Departed, which runs to a tidy two hours, is Scorsese's third consecutive project with DiCaprio, the 31-year-old actor with whom the director has developed close bonds. "There's an inner story going on with DiCaprio that somehow I was able to tap into, which is similar to what I feel," he says. "Yes, it will always be somewhat different because I'm older, but hopefully we can make a few more pictures together."

For Scorsese aficionados, however, the good news is that Scorsese is reuniting with Robert De Niro for a future project, an autobiographical story "about our lives together", which they will resume writing when De Niro finishes his own directing job, The Good Shepherd, starring Departed star Matt Damon.

De Niro and Scorsese grew up in the same neighbourhood together and the project will look at their experience of the Italian diaspora in America - not a million miles away from the Irish of Boston.

"Even though my father was born on Elizabeth Street, in 1913," says Scorsese, "he is still very much part of the Sicilian village that my grandfather came from. It was very hard to become fully American." Unusually for Scorsese, he pauses. "Whatever that is."

The Departed opens on 6 October.