You might expect John Williams to be a tortured soul. The world's most successful writer of film scores - including Jaws, Indiana Jones, Star Wars, and ET - he also produces occasional concert pieces. So is he a frustrated Beethoven forced to earn a living in a battery-hen world?

Williams will have none of it. "I think of myself as a film composer," he says in his measured, professorial way. "I'm not a frustrated concert composer, and the concert pieces I've done have been a small part of my work. What I've sought there is instruction, variation from the demands of film and relief from its restrictions."

The composer is 70 this year, as productive as ever and apparently beyond ego. That must come when you have won five Oscars and been nominated 30 or so times. But he accepts that traditionalists see film scores as a very inferior form of classical music. "We have to be hopeful," he says, "that if there is a musical genius in the future, that individual is someone who has a connection with film and doesn't regard the old division between fine arts and media arts as rigidly as we do."

Williams, New York-born and Los Angeles-based, is in London to record the anthem he has written for Friday's opening of the Winter Olympics in Salt Lake City and the music for the latest episode of the Star Wars cycle. He is already booked up for the next two years - Harry Potter II and III, Minority Report, yet another Star Wars movie and Catch Me If You Can. What if he gets an idea for a string quartet whilst in the middle of the latest Star Wars or Harry Potter score? Would he risk running late?

"It doesn't really work that way," he says. "Once in a while I will make some sketches for something that may be a future project, but rarely. I'm focused on what I have to do that particular day. The demands of the schedule are so great that you have to keep pace with it."

Isn't this Trollopian approach to writing music - each quarter-hour accounted for - the opposite of the classic notion of inspiration? "The romantic notions of how inspiration comes are just that - notions," he says. "Composing music is hard work. Any working composer or painter or sculptor will tell you that inspiration comes at the eighth hour of labour, rather than as a bolt out of the blue. We have to get our vanities and our preconceptions out of the way and do the work in the time allotted."

Placido Domingo has asked Williams to write a work for the Los Angeles Opera. Surely this will be an irresistible opportunity to test a new musical muscle? He is oddly cool about the project. "It would take a couple of years and I'm not sure whether I will have the time, but I might do it. It'll depend on the subject and how I feel about the libretto he is preparing. Even if I do say yes, I would do it with a sense of reticence in that I'm not a theatre person or particularly a vocal composer."

Why not do it instead of more Star Wars? "Star Wars is something I would like to complete if I can. I've enjoyed adding tunes to the collection of melodies and melodic identifications that go with the characters. But I would also say that there are sometimes commitments in life that are the result of relationships that are in place."

Star Wars guru George Lucas is one of those relationships; Oliver Stone another (Williams wrote the scores for Born on the Fourth of July, Nixon and JFK). But the central relationship of his working life is with Steven Spielberg. He has written the score of every Spielberg film except The Color Purple; his spine-tingling music for Jaws in 1975 took him into the big league after 20 years of solid film and TV work; and three of his Oscars were for Spielberg movies - Jaws, ET and Schindler's List.

He likens his relationship with Spielberg to a marriage - and is clearly still besotted. "Steven is a very warm, sweet man. The success of his films is not so much the result of craft and artifice. Rather, it's because of his basic humanity. He's a fantastic person, and that is what's delivered to the audience. It's been a very happy relationship over 30 years, though I take nothing for granted: there are a lot of composers in the world and he may wish to use some others." Somehow, though, you rather doubt it.

Williams was born with a score in his hand: his father was a musician with the CBS Radio Orchestra in New York and later with 20th Century Fox in Los Angeles. He studied composition privately and went to the Juilliard school in New York to study piano. He worked as a jazz pianist in New York before returning to Los Angeles to play with the Columbia Pictures Orchestra. He started doing orchestrations, then TV scores and finally films.

It's hard to tell whether Williams has a life beyond composing. His antidote to work appears to be more work - conducting the Boston Pops Orchestra. He was its chief conductor from 1980-93 and still appears with the orchestra about 20 times a year. "It's a familial kind of connection," he says. Movie-going is less of a preoccupation. "I live with films every day - I have in my working room a copy of the films that I'm working on and I look at the scene I'm doing half a dozen times that day. So it's not my habit to go home at night, pack up, and go out and watch a movie."

Of Spielberg's movies, he picks out Close Encounters - "it was more than just Cellophane going through a projecting machine, it had a kind of life" - and Schindler's List, which he says was "one of those rare occasions where you can run the whole film, stop it anywhere and find something quite beautiful in it".

"When he showed me Schindler's List," says Williams, "I was so moved I could barely speak. I remember saying to him, 'Steven, you need a better composer than I am to do this film.' And he said, 'I know, but they're all dead.' " That's a joke. Probably.

· American Journey, which includes the theme for the Winter Olympics, is released today on Sony Classical. On March 4 Sony will release a disc of Williams's cello music. The next episode of Star Wars will be released in the spring.