On stage, he appeared in the original 1988 production of Timberlake Wertenbaker's play Our Country's Good. He was nominated for a Laurence Olivier Award in the category of Best Supporting Actor in 2000 for his role in Juno and the Paycock at the Donmar Warehouse. He also appeared in a new play by Conor McPherson, The Seafarer, at the Royal National Theatre.
He has performed in a large number of television productions, including guest roles in episodes of series such as The Black Adder where he played "Sean the Irish bastard" (1983), Bergerac (1988), Sharpe (1994) and Doctor Who: "The Idiot's Lantern" (2006). He has also had major roles in more prestigious one-off productions and serials, including several instalments of the BBC's The Complete Dramatic Works of William Shakespeare (1982–83), most notably as Richard, Duke of Gloucester, later Richard III of England in Jane Howell's repertory treatment of the Henry VI plays and Richard III. He appeared as one of the unnamed "mysterious men" haunting the imagination of Michael Gambon's hospitalised writer in Dennis Potter's acclaimed 1986 serial The Singing Detective, and has featured in several costume dramas, including Stephen Poliakoff's The Lost Prince (2002, as David Lloyd George), an adaptation of The Hound of the Baskervilles (2002, as Barrymore), a TV adaptation of The Other Boleyn Girl, (2003 as Thomas Cromwell), Anthony Trollope's adaptation He Knew He Was Right (2004, as Bozzel), and Russell T. Davies's Casanova (2005, as the prisoner in the cell next to Casanova's). In 2006, Cook appeared as Kenneth Williams' agent Peter Eade in the BBC biopic Kenneth Williams: Fantabulosa!. He also played the role of iconic Victorian engineer Isambard Kingdom Brunel in the BBC's Seven Wonders of the Industrial World.
He has played Napoleon Bonaparte twice, in his 1994 guest appearance in Sharpe and again in the 2000 feature film Quills. Other film roles have included parts in The Cook, the Thief, His Wife & Her Lover (1989, as Mews), Secrets & Lies (1996), Topsy-Turvy (1999, as Richard D'Oyly Carte), Chocolat (2000), Charlotte Gray (2001), 24 Hour Party People (2002, as Derek Ryder), Thunderbirds (2004, as Parker) and Hot Fuzz (2007, as George Merchant). Cook also appeared in Feeling Good, a short film written by Dexter Fletcher and directed by Dalia Ibelhauptaite.
Cook has also acted in radio drama. In 2007 he played the part of confidence trickster Captain Wragge in a BBC Radio 4 adaption of the Wilkie Collins novel No Name. In July 2007, he played the part of Kris Kelvin, the protagonist psychologist on the BBC Radio 4 adaptation of Solaris, Stanislaw Lem's novel.
