Duncan's film credits include Prick Up Your Ears (1987), The Reflecting Skin (1990), City Hall (1996), An Ideal Husband, Star Wars: Episode I – The Phantom Menace, Mansfield Park (all 1999), Under the Tuscan Sun, AfterLife (both 2003), Starter for 10 (2006), Tim Burton's Alice in Wonderland (2010), About Time (2013), Birdman (2014), and Blackbird (2019).

[5] Her mother was affected by Alzheimer's disease and died in 1994; she inspired Sharman Macdonald to write the play The Winter Guest (1995), directed by Alan Rickman, which he later adapted as a film.

[3] She became friends with future playwright Kevin Elyot, who attended the neighbouring King Edward's School for boys, and followed him to Bristol, where he read Drama at university.

[9] She made her breakthrough on Top Girls by Caryl Churchill, staged at the Royal Court in London and later transferred to the Public Theater in New York, Her performance as Lady Nijo, a 13th-century Japanese concubine, won her an Obie, her first award.

She was replaced by Glenn Close for Dangerous Liaisons — Stephen Frears's film of the play; similarly John Malkovich was selected for the role of Valmont instead of Duncan's co-star Alan Rickman.

[13] In 1994–95, she performed for a second season with the RSC in A Midsummer Night's Dream, in the double role of Hippolyta and Titania, replacing Stella Gonet from the original production cast.

[14] She went on tour in the United States with the rest of the cast, but back and neck pains forced her to be replaced by Emily Button from January to March 1997.

[15] Impressed by her performance in David Mamet's The Cryptogram (1994), Al Pacino asked Duncan to play the role of his wife in City Hall (1996) by Harold Becker.

She also starred in the original London run of Polly Stenham's play That Face at the Royal Court co-starring Matt Smith and directed by Jeremy Herrin.

In October–November 2010, she starred in a new version by Frank McGuinness of Ibsen's John Gabriel Borkman at the Abbey Theatre, Dublin alongside her Liaisons dangereuses co-stars Alan Rickman and Fiona Shaw.

[20] Alan Bleasdale asked Duncan to appear in his first work for television after ten years of absence, The Sinking of the Laconia, aired in January 2011.

[23] She also appeared as Home Secretary Alex Cairns to Rory Kinnear's Prime Minister in "The National Anthem", the first episode of Charlie Brooker's anthology series Black Mirror.

In February, she returned to the West End in Noël Coward's Hay Fever with Kevin McNally, Jeremy Northam and Olivia Colman, once again under the direction of Howard Davies.