Micaela Carosi's understudy seizes the day

The star falls sick, the understudy takes over and becomes a star: one of the nice things about opera is the way this time-honoured tradition repeats itself year after year. The latest example was when the much-hyped Italian soprano Micaela Carosi suddenly discovered a few days before curtain-up – suddenly? – that her advanced pregnancy would prevent her starring in Covent Garden's Aida.

Hardly anybody in London had heard of her understudy, the Ukrainian soprano Liudmyla Monastyrska, and this was one hell of a part to step into at short notice. "Yes, I was nervous," she says "But as soon as I began to sing I felt OK." You bet she did: her sound was so full and majestic, her presence so powerful, that she sang everyone else into the shadows, and her reviews were a unanimous rave. "But what really made me glad was the people who came to the stage door afterwards, and thanked me for singing with such heart."

Next Tuesday at Covent Garden she opens as Lady Macbeth in Verdi's version of the Scottish play, and we shall see a different side to her. "I want people to understand Lady Macbeth's predicament – she's a very complex character. She knows her final punishment comes from on high."

Married to a tenor and raising her teenage children in Kiev, Monastyrska is herself – if her agent's word is anything to go by – a complex character, but in the best sense of the term. She weighs her words before she speaks, considering all sides of every question; she's a fine pianist, a voracious reader of Tolstoy, and offstage a natural comedienne. Co-star Simon Keenlyside will have to raise his game to match.

'Macbeth', Royal Opera House, London WC2 (020 7304 4000) 24 May to 18 June