For MacMillan, the perfect opera subjects are probably Shakespeare plays or episodes from Greek or Teutonic mythology - something archetypal, ritualised. Seeing John Clifford's play, on which the opera is based, at the Traverse Theatre in Edinburgh in 1989, he was gripped. "I was genuinely moved by the human dimension - the personal relationships - when I saw the play; the violence, the extremes were there, but expressed through words, not gratuitously. I am sometimes disturbed by a lot of contemporary art - there's a nihilistic glee about gratuitousness." The play, based on the work of the 16th-century Portuguese poet, Antonio Ferreira, attempts to impart a classical detachment and poise to messy historical events, and this is the approach that MacMillan pursues in his operatic version.
Composer and director have worked together closely from early on in the project. Meetings every couple of months have avoided some of the obvious dangers involved when a producer arrives to "interpret" the cherished brainchild of a composer. Having a live composer does put a different complexion on things. In fact, Moore, a specialist in new operas, has "never worked with a dead composer", as he puts it.
"This is the first contemporary opera where I haven't had to change anything drastically. The director's role is ultimately interpretative, but it's vital to bring something to the piece consistent with what the composer wants. Each piece is different: you can't just do the blue plastic thing - you know, 'Darling, I've got a thing about blue plastic, I'm sure we can get it in somewhere.' And of course, as it's a new piece, clarity is essential. And you've got to engage the audience on an emotional level; too many people think of contemporary opera as somehow a cerebral business."
Just who is this audience for contemporary opera?"I hope people from many different musical backgrounds would want to come," says MacMillan. "There's the people who are always interested in new work, who are very knowledgeable and critical; then there's a wider classical music audience, who hopefully still take an interest in new operas, and I'd like to think a theatre audience would be interested, especially as the play was a big success here in Scotland."
And how to get this grim tale of a prince's mistress, a foreigner in a land at war, despised, rejected and eventually put to death, across to any audience? Moore is very firm on the need to avoid sensationalism - the dreaded "naked nuns syndrome". "It's too easy, too superficial, to go for sensational gestures. This piece is a cry from the heart for love - a kind of Blakean struggle between innocence and experience. Ines, the central character, manages to maintain this innocence despite everything. And the venal characters, like the 'villain', Pacheco, get to share with the audience why they are like that."
Ultimately, as in much of MacMillan's work, the religious dimension is crucial - the piece starts with liturgical music, a setting of the Stabat Mater, and this thread runs right through. "If the religious element hadn't been there, it would have been a different piece altogether, and not one I would have set," explains the composer. "But I'm keen not to be seen as imposing a particular religious outlook - you know, people saying: 'Here comes that Catholic composer again.' I see this work as dealing with universal truths. In that age, in that world, church and state were inextricably linked. And even now, I feel that the 'spiritual' is not to be divorced from everyday life. You've got the so-called 'holy minimalists' - Part, Tavener - who write beautiful music, but give the impression that the sacred is somehow opposed to the everyday. For me, you can only realise that dimension by going through the mundane, through the abyss, even."
Moore endorses this: "After all, opera is a kind of religious experience, isn't it? You've got to be holistic. Who was it who said: 'Laborare est orare'? Even the most mundane things can be offered up, and we've all heard about the banality of evil. A Christian point of view doesn't exclude other ways of expressing the same thing. Whether you're a devout Buddhist, a Moslem, or whatever, there's a salvation for you somewhere."
And what salvation is there at the end of this particular dramatic vision, which is undeniably one of tragedy and evil triumphant?
"There's no happy ending," admits MacMillan, "but there is this glimmer of goodness, of hope. Ines represents an ideal - that there is another way; the little child who is the only person who can speak to her ghost, at the end, is perhaps a seed of hope. People will still come out feeling the full force of the violence and hate. There is no great point-making, no ramming home of a message - just the suggestion, the hint, that there is always an option for good."
n 'Ines de Castro' at the Festival Theatre, 23, 25 Aug, 7.15pm. Booking: 0131-225 5756