Sometimes life mirrors art in the most devastating way. Last Friday the conductor Richard Hickox was in the midst of exhausting final rehearsals for English National Opera's new production of Vaughan Williams's harrowing opera Riders to the Sea - a work that is a mere 40 minutes long, yet saturated with death and with people struggling to come to terms with death. On Sunday Hickox himself died of a suspected heart attack. He was just 60.

For Fiona Shaw, this introduction to the world of opera could hardly be more traumatic. The 50-year-old Cork-born actress is making her opera-directing debut with Riders. “I went to a concert Richard was conducting in Cadogan Hall a few days ago,” she says. “He told me about his frantic schedule, and I said to him: ‘How on earth do you keep going?' He replied: ‘Enthusiasm breeds its own energy.' I think those words summed up his whole attitude to life and music. He gave so much. We are in a state of shock. It's as if a hole in the ground has appeared.”

But the show will go on, as Hickox would certainly have wished. ENO's music director, Edward Gardner, will conduct. And suddenly the grief portrayed in Riders will have an unexpected and tragic immediacy. “This year has been full of death in my life,” Shaw says. “But I thought that, in late November, I was finally out of it. Now this terrible news.” Could these performances of Riders prove consolatory, or even cathartic? The subject matter is certainly apt. “We're meeting the central figure, the mother Maurya, at the very end of her tragedy,” Shaw says. “Almost everything terrible has already happened to her. What we watch is the straw breaking the camel's back.”

That's true. Based on J.M. Synge's 1902 play, the opera is set in an impoverished fishing community on the Aran Islands, off the west coast of Ireland. Maurya has already lost her husband and four sons to the sea. A fifth, Michael, is missing, presumed drowned. Forty minutes later, the dripping body of the sixth and last, Bartley, will be carried in. It's not a show to undertake, or to experience, in anything other than the most serious frame of mind. The subject is grief, raw and unmitigated. Even before Hickox's death, Shaw was already drawing on her own experience of mourning a brother.

“My dear brother was killed some years ago,” she says. “What I realised is that if you are close to someone who died, you suffer grief, of course - but also people close to you suffer grief because of your grief. That can make you the object of revulsion as well as sympathy. Maurya's daughters have to endure her grief, so perhaps can't develop their own grief properly. That's what often happens to siblings in families where the boy dies.”

In the opera, Maurya has a terrible premonition of the dead Michael leading his one surviving brother into the sea. Shaw, controversially, will depict Michael actually appearing, portrayed by an actor - as if still haunting his mother's consciousness. “I saw my own brother after he died,” Shaw says. “I was walking in fields by Stratford-upon-Avon and I saw him - at least a young blond boy exactly like him - by the river. I knew that if I walked up to him he would change into someone else. But from a distance it was him.”

And Maurya's vision of the dead brother leading the living one to his death? “Possibly she is projecting her need to be freed of her final son, in order to be permanently released from the terror of the sea,” Shaw says.

“I read a wonderful book this summer: Darian Leader's The New Black: Mourning, Melancholia and Depression. It's about how we haven't properly developed our processes for dealing with grief, and coming back to non-grief. It's not just the grief. You also have to deal with the shared memories. My dog was killed earlier this year. I feel that all those walks I did with him belonged in the shared memory of me and him - and now he's taken those shared memories with him. We lose lumps of ourselves when other people grow ill or die. And that can remove some of our own identities. My father is ill at the moment, and I suddenly have had to shift my own identity - put my own life and work on hold - in order to deal with something much more important.”

Shaw and the Irish artist Dorothy Cross have set the action of Riders, she says, “literally among the rocks, with these poor, subsistence-level people clinging to existence like crabs”. She is also emphasising the wild forces of Nature invoked in the opera by prefacing it with an atmospheric Sibelius vocal work called Nature Spirit, and then with specially composed sounds of whooshing and Celtic keening by the composer John Woolrich (“I told him it must sound like nothing on earth!”).

Yet she denies that Riders is a specifically Celtic work. She detects the inexorability of Greek tragedy - and also something closer to home for Vaughan Williams, composing in the 1920s. “After the First World War the overwhelming number of dead meant that a whole generation couldn't deal with grief,” she says. “In choosing Synge's play, Vaughan Williams may have been responding to that. He may have noticed people blocking up grief, or diminishing it.”

Shaw's belated big-screen fame as Harry Potter's Aunt Petunia, and the unsought gossip-column exposure stemming from her relationship with the actress Saffron Burrows, have not deflected her from the stage. Before preparing Riders she was touring in Beckett's Happy Days, and after the opera opens she will dash to Paris to speak a prologue in a production of Purcell's Dido and Aeneas by her longstanding collaborator Deborah Warner. Next year she's back at the National in London to play in Brecht's Mother Courage.

So how difficult has she found adjusting to the very different theatrical rules of opera? “How difficult? Have we got time to do a book? What I have realised is the obvious thing: that the music defines a lot of your choices as director. If I were doing Synge's spoken play there would be lots of moments where I would stop the dialogue and let minutes go by. Here, though, Vaughan Williams sets the pace.

“I'm well out of my comfort zone. I have an L-plate on my back. Having said that, a week ago I was having some of the happiest times I've ever had in a rehearsal room, with a wonderful cast playing scenes brilliantly. Of course, the mood is so different now.”

Riders to the Sea opens at the London Coliseum (0871 9110200) on Thurs

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