As the willing bride of a serial wife-murderer, Judith embodies a quintessentially Freudian fantasy: Bartok's Duke Bluebeard's Castle plugs directly into psychoanalysis. And when the curtain rises on John Macfarlane's richly suggestive set, that exploratory feeling will be reinforced. This one-act, two-character opera may last only an hour, but it's a huge challenge for the mezzo who stars in it. In Petra Lang, the Royal Opera have found a singer well equal to the challenge.
Lang has an arresting take on what Bartok was up to. "When I first studied the role," she says, "my family went through a difficult time, because it put me in a very bad mood. Judith's behaviour is the perfect way to destroy a relationship. When you really love somebody, you take him or her as they are: you leave them with their secrets intact. But Judith makes Bluebeard open his soul - she decides she can be his saviour, but what she does is cruel. I found this difficult to accept."
This idea, she argues, is cleverly dramatised by Bartok, who she is sure hated women at some subliminal level. "He gives wonderful bel canto lines to the baritone, but for the poor mezzo-soprano it's difficult even to find the right pitch. Her music is very aggressive, and often goes against the prevailing key."
That, says this forthright German singer, is his way of showing what a negative force she is. "Bluebeard will say something in G major, and she will reply in the pentatonic scale in A flat - going a semitone against him. They never come together musically. The orchestra is mostly very loud, and is very thick at the lower part of the female register - no female voice can cut through it. And that is his way of saying that though she is negative, she is also very weak."
But finding her way through this personality - behaving in a way she herself never would - became, she says, a privileged exploration. Like getting inside the mind of a murderer? "Exactly."
Tomorrow to 17 June (020-7304 4000)