A Lady Macbeth dressed to kill

Sian Thomas gives the inside story on Shakespeare's ultimate power couple

For her role as the murderously ambitious Lady Macbeth, Sian Thomas sports long blond curls and, at one point, a powder-pink party dress that makes her look endearing rather than evil. "I am not playing a fiendish queen, but a vulnerable human being," Thomas says. "Part of Lady Macbeth's problem is that she has nowhere to put her energy and ambitions except through her husband, because she comes from a society where women have no power. She is an ordinary woman with desires. She just goes too far."

Thomas's wardrobe is by Tania Spooner. The costumes include a full-length tweed dress - "worn when Lady Macbeth hopes to become queen" - and an extraordinary coronation robe. "It's a low-cut, white satin outfit with a beautiful brocade cloak," says Thomas. "I'm laden with these jewels and necklaces and a crown, and have a tight choker round my neck. But somehow I am imprisoned in it. Lady Macbeth gets what she wants, yet she is not as happy as she ought to be. This costume reflects that beautifully."

The RSC's associate director, Dominic Cooke has chosen to locate the play in a non-specific period, although Thomas describes Robert Innes Hopkins's set as having "a late 19th-century Russian-esque feel to it".

Greg Hicks plays her consort as a man fraught with anxiety, his resolve weakening at the prospect of murder. Thomas's Lady Macbeth cajoles her spouse with reproaches, fury and seduction. "They love each other," she says. "They are devoted. They never betray one another. They are not mean and cold like some of Shakespeare's other villains. They have been together a while. All is not quite well in terms of having children - though she obviously had a child at some point, because she says, 'I have given suck, and know/ How tender 'tis to love the babe that milks me.' Perhaps their relationship is rekindled by the murder plan."

Lady Macbeth may be the pinnacle of tragic roles for women in Shakespeare, but Thomas has had "plenty of other great roles". Recent credits include Gertrude in the RSC Hamlet and Esther in Arthur Miller's The Price at the Apollo with Warren Mitchell.

"Another fantastically dramatic part was Hedda Gabler, directed by Michael Boyd," Thomas says. "I also played Helena in Uncle Vanya, directed by Peter Egan and Kenneth Branagh, and one of my favourite parts was at the National as Célimène in Molière's The Misanthrope - that is a gift of a part." Thomas was also nominated for an Olivier award for her cameo in Up for Grabs in 2002 with Madonna.

Macbeth is, of course, usually referred to by its casts as "the Scottish play" because of the superstition that it is bad luck to speak its name. Thomas has no truck with that: "It is too much effort not to mention the play by name when you are in it. I ignore the superstition."

'Macbeth', Albery, London WC2 (0870 609 1110), to 5 Mar