Preview: Il Re Pastore, Linbury Theatre, ROH, London

When the American soprano Katie Van Kooten stepped into Angela Gheorghiu's role in the Covent Garden La Rondine last year, people thought this young singer was taking a risk.

They needn't have worried, any more than when she sang Pamina in Die Zauberflöte. Her voice is so beautiful, and her art so effortless, that whatever she does she commands the stage. And so it will doubtless be again, when she sings the shepherd-king Aminta in Il Re Pastore, the opera Mozart composed when he was 19 years old.

"The role was written for a castrato," she says, "but it's very comfortable for me. Some people get upset when you say that one of Mozart's works is not his best, but it doesn't detract from his genius to say that, when he was younger, he hadn't found his mature form. This was written for entertainment only, and a lot of the story is just to fill in time between the musical numbers. But my big second-act aria is a masterpiece - it reminds me of "Porgi amor", the Contessa's lament in Figaro. It has a lovely line demanding great legato, and it sounds so simple, which is the whole beauty of Mozart."

Given that Kooten is tall and curvaceous, how will she cross the gender divide? "I'm not strapping anything down, I'm wearing breeches and billowy shirts - I have to be comfortable, and able to move."

She will be helped, she thinks, by the fact that Aminta has not really settled into himself at the start of the story. "He's still an awkward boy. The physicality required by the part is ideal for me. He's the one character who has a journey in this opera. As the story develops, he changes from being a boy to becoming a man. So gradually he will become more direct and forward in his posture." Kooten squares up to me. "As opposed to the way a woman is more circular in her movements." A demure gesture. "It's about using your weight in a different way."

Tomorrow (020-7304 4000; www.royalopera.org)