[12] Duncan's stage credits have included plays by William Shakespeare (as Rosencrantz in Hamlet, Gonzalo in The Tempest, Gower in Pericles, Time in The Winter's Tale, Friar Laurence in Romeo and Juliet, and Banquo in Macbeth), George Bernard Shaw (in Village Wooing and as Frank in Mrs. Warren's Profession), and Noël Coward (in Tonight at 8.30 and as Garry Essendine in Present Laughter).

He has played Trepan in The Master and Margarita, Governor Bellingham in The Scarlet Letter (Chichester Festival Theatre), and has performed in many musicals, including The Boy Friend as Alphonse, Privates on Parade as Charlie, Happy End as The Professor, Kiss Me Kate as Hortensio, Sweet Charity as Vittorio Vidal, Anything Goes as Sir Evelyn Oakleigh, and Company as Harry.

[13] In 1987, Duncan became an associate artist of the Crucible Theatre, Sheffield, where his many roles included appearances in The History of Tom Jones, as Canon Chasuble in The Importance of Being Ernest, Antonio Salieri in Amadeus, Courtling in the British première of The Park by Botho Strauss, and Simeonov Pischik in The Cherry Orchard.

Among their dozens of plays performed across the UK, he has written and directed The Revels of Gargantua in Exile Part II (1974), Kino Tata (1976), Milady's Silver Musick (1977), The Amusing Spectacle of Cinderella and Her Naughty-Naughty Sisters (1977), Stringgames (1979), A Cow.

His productions there included The Nose, The Adventures of Pinocchio, Kurt Weill and Bertolt Brecht's Happy End, A Fool and His Money (Scottish Opera and Birmingham Repertory Theatre), The Cabinet of Doktor Caligari (Lyric Theatre), Time and the Room (Edinburgh International Festival, Krapp's Last Tape with John Neville, and Endgame (Weimar Festival).

Duncan was Associate Director on both Steven Pimlott's production of Twelfth Night at the Sheffield Crucible and the Pet Shop Boys' 1991 World Tour.

The show starred Simon Russell Beale, Daniel Evans, Julian Ovenden, and Judi Dench who sang Sondheim's "Send In the Clowns",[15] a piece originally written for actress Glynis Johns.

Premiering on 12 October 1988, Duncan choreographed Stephen Lowe's Divine Gossip at the Royal Shakespeare Company's Pit Theatre.

Duncan has written the libretti for and directed two short operas: Three Really Good Tea Parties with composer Jonathan Dove at the Salisbury International Arts Festival and Warning Bells with writer Jeremy Sams in 1989, Dartington.

These include L'heure espagnole (1989), Gianni Schicchi (1990), The Thieving Magpie (1992) Orpheus in the Underworld (1992), The Adventures of Pinocchio (2007), and A Midsummer Night's Dream (2008), and Yolanta (Edinburgh International Festival).

[18] For Munich's Bavarian State Opera, he directed the Opernwelt award-winning Xerxes (1996), La clemenza di Tito (1999), The Rake's Progress (2002), and Die Entführung aus dem Serail (2003).