Thelma Schoonmaker's name may not be as well known as that of Martin Scorsese, but as film editor on Gangs of New York and classics such as Raging Bull and Goodfellas, she has been the final shaper of the celebrated director's vision for over 20 years. She talks to Donald Clarke about the cutter's role

Of all the participants in the film-making process, the editor is the one whose contribution is hardest to disentangle from that of the director. Audiences may search the credits to discover who designed those lovely hats, or who wrangled those awesome buffalo, but if they admire the rhythms of the editing, as often as not the man in the jodhpurs will get the credit.

During a warm, amusing masterclass, arranged by Screen Training Ireland in Dublin's Westbury Hotel, Oscar-winning editor Thelma Schoonmaker does not seek to right this wrong. She is happy to eulogise the art department, the sound designer and the musicians, but seems reluctant to focus on her own input.

Schoonmaker's mutually exclusive working relationship with Martin Scorsese, which has endured since their first major professional collaboration on 1980's Raging Bull right up to this year's Gangs of New York, has made her one of the few editors the average film enthusiast can name, but has also pigeon-holed her as "Scorsese's editor". Does she not long to investigate a separate identity, to discern how much of their success is the result of her influence?

"Not really," she says, sipping a beer after her lecture. "I'm carrying more of the weight now than I used to. I'm doing more of the editing myself as Marty gets more heavily involved with other projects. I don't care who gets the credit, because it's so rewarding. My goodness, it is so challenging doing this, I couldn't bear any more challenges."

Though Scorsese and Schoonmaker have the same fearsome intelligence and love of film, otherwise, their characters could not be more different. While he is famously fast-talking and energised, the 62-year-old editor has the gentle, old-fashioned manners of a kindly aunt.

"Oh yes, but I think that's why we get along so well together," she says.

"I'm very even tempered, very optimistic and Marty's much more complex. I go through life rather easily; he finds everything difficult. He's like an open wound, and that's why I think he's such a great artist: he feels things more."

Suddenly, the door of the meeting room bursts open and a head pops in: "Thelma, it's Martin on the phone. He says it's urgent." Schoonmaker apologises and pads out to the hotel lobby to discuss a series of excerpts they are editing together for a Scorsese tribute. Even with several thousand miles of ocean between them, they remain indivisible.

They first met in the late 1960s when Schoonmaker stumbled into the business after answering an advertisement looking for people to assist in the editing ("butchering" she now says) of movies for TV.

The political science graduate, who originally intended to be a diplomat, unexpectedly found herself part of the most exciting generation in post-war American cinema, a generation that included Francis Ford Coppola, George Lucas, Paul Schrader and Scorsese himself.

"That generation was not well organised in the editing room," she says, with a slight hint of disapproval. "They could shoot it, no problem. But organising was not their thing. Women do not like to hear this, but I think that women do have a better ability to organise. They have been trained for it. When you're cooking or taking care of babies you learn how to manage your time better. They were terrible in the editing room: always breaking the film. I think being a woman did help."

It is true that there are many more women working as editors than as directors, but I am slightly appalled by what she says. She seems to be casting the female editor in the role of a housewife who cooks up the raw meat that the big strong male director brings home at the end of a hard day.

I imagine a lot of women would be unhappy with that analogy.

"Oh well. I have no problem with it," she laughs. "There are a lot of male editors of course. So it's not exclusively a female thing."

Scorsese and Schoonmaker worked together on the director's first feature Who's That Knocking At My Door? and that dated, 1970 paean to mud and marijuana, Woodstock. But union problems prevented her from officially contributing to any of Scorsese's great films of the 1970s (though she did an uncredited nixer on Taxi Driver).

However, their lives remained inextricably knotted together. When his then editor Marsha Lucas decamped for northern California to work with husband George, Scorsese helped get his old friend a union card and brought her back into the fold for Raging Bull. And, perhaps inevitably, it was he who introduced her to her future husband Michael Powell, the great English director of such films as A Matter of Life and Death and The Red Shoes.

"Marty was in Edinburgh receiving an award for Taxi Driver, about 1978 I guess, and they asked him who he would like to present it, so he said Michael Powell. They were amazed: 'Michael Powell? Nobody knows who he is or where he is.'" Powell, now recognised as perhaps the greatest British director never to be lured to Hollywood, had been cast into obscurity after his controversial 1961 film Peeping Tom opened to howls of disgust.

"He was living in total poverty in his cottage in the Cotswolds," she explains. "He was in a terrible state. He was heating the place with wood that he chopped himself, and he was 73 at this stage. He had sold all his paintings, all his first editions. But he would never describe it that way to me. He was never bitter."

Meeting Scorsese and Schoonmaker completely transformed Powell's fortunes. Happily surrounded by younger acolytes, he fell into teaching and by the time of his death in 1990 his reputation had been re-established. "He said it was like waking up from a nightmare," she says.

Schoonmaker's girlish delight as she describes life with Powell is terribly moving: "I'd come home from work for lunch and he'd have arranged the fruit on the plate in some startling fashion, like a Picasso still life. Then I'd come back in the evening and he'd have rearranged it again. He was always enjoying life. He would never waste a second. But you've no idea how exhausting that is to live with, even though I was 45 and he was 73."

Both Scorsese and Powell threw away the templates the studios expected them to use. But Scorsese has somehow managed to stay on the right side of the money men whereas Powell ended his career alienated from the British movie establishment.

"Michael always knew that when he went out on a limb, particularly with Peeping Tom, he might get that limb sawn off. And he was prepared to accept it when it happened," she says. "Marty, on the other hand, has learned to walk a dangerous tightrope with studios. He likes a lot of the studio heads and he knows how to bring them along with him, whereas my husband would just insult them and walk out of the room. 'Oh, that's a stupid idea,' he would say.

"Now Marty would never do that. He would always find a way of putting it that wasn't offensive. It is remarkable, I've been in meetings where I couldn't believe what I was hearing from the studios. But he would just say, 'Well, that's a very, very interesting idea, but I don't think I could make that movie'." Which brings us neatly to the long-delayed Gangs of New York. Much has been made of the alleged battles between Scorsese and Miramax head Harvey Weinstein over the final cut of the picture.

"Harvey has very strong opinions," she says. "And we fought about a lot of them, but we all agreed in the end. It was an interesting way to work, and we've never worked that way before. But it wouldn't have been made without him, so we're grateful for that. It's always difficult with studios, frankly."

What about the widely held belief that she and Scorsese would have preferred the film to be at least 20 minutes longer?

"Well that's not true," she says. "If we had more footage, then yes. If we had been able to shoot more material establishing Leonardo DiCaprio's character a bit better, then yes. More scenes with Liam Neeson as the father and the boy perhaps. So, if we had shot more, then we would have liked to put that in."

This may come as a disappointment to those of us who felt that, like Heaven's Gate or Blade Runner, the film would be only be fully realised when released as a director's cut.

"No. Marty doesn't believe in director's cuts. He feels you should fight for what you believe in during the editing stage and then live with the consequences. You shouldn't think, 'Oh, it's OK to cut that because I can always put it back later'. He'll fight to the death. He would go to the press if he felt he was not getting the film he wanted."

So, we can't get her to badmouth Harvey? "Well, we got what we wanted and that's what matters." She should have been a diplomat after all.