The company's permanent base became London's Theatre Royal Stratford East in 1953, the opening production being Twelfth Night with Cooper as Malvolio and Harry H. Corbett as Sir Andrew Aguecheek.

[1] Cooper played Geoffrey Fisher, the stern father of the eponymous antihero Billy Liar, in the original West End stage production in 1960.

According to Tom Courtenay, who took over the role of Billy from Albert Finney, immediately prior to his entrance in Act One, Scene One, while waiting in the wings, Cooper would regularly mime taking 'the most enormous shit', which would involve removing his braces, lowering his trousers and making (in)appropriate noises, a routine he timed impeccably, so that his stage entrance would immediately follow hoisting his trousers and braces back into place.

[5] His wife, Shirley Jones (married 1955), who worked as a Theatre Workshop costume assistant, did not like his absences during the evenings, and Cooper himself found eight performances of Billy Liar a week to be a strain.

[8] Among Cooper's other television credits are Vicky and the Sultan, Danger Man, Z-Cars, Dixon of Dock Green, No Hiding Place, Doctor Who (in the serial The Smugglers), Angel Pavement, Softly, Softly, The Avengers, The Saint, Randall and Hopkirk (Deceased), The Troubleshooters, Steptoe and Son, A Family at War, Doomwatch, Public Eye, Budgie, Bless This House, Sykes, Rising Damp, The New Avengers, Some Mothers Do 'Ave 'Em, All Creatures Great and Small, Poor Little Rich Girls, Juliet Bravo, When the Boat Comes In, Terry and June, Taggart, Casualty and as Walter Petigrew in Heartbeat.