The Score: Brian Cox is magnificent as he returns to the stage

Brian Cox was on outspoken form at Cheltenham Literature Festival the other week, when he seemed to channel Logan Roy, the scary-sweary mogul who dominated Succession and made Cox – now 77 – a Golden Globe-winning force to be reckoned with.

Drawn on the subject of the Winston Churchill biopic Darkest Hour, Cox – who played the wartime prime minister in another 2017 film – pronounced it “a piece of s---”, growling: “They had [Gary Oldman] going into the subway [London Underground] and talking to people. You think ‘What the f---?!’’”

A certain irony about his outburst becomes apparent when watching The Score, which sees Cox return to the UK stage for the first time since 2006, ahead of a hot-ticket West End run in Long Day’s Journey into Night. Penned by the actor-playwright Oliver Cotton, it imagines the encounter between J S Bach (Cox) and Frederick the Great (Stephen Hagan) that took place at the Prussian king’s palace in 1747, three years before the composer’s death.

Roughly what occurred is known: Bach was challenged by Frederick to improvise a three-part fugue on a figure seemingly designed to be as resistant to counterpoint as possible. The composer completed the task, only to be asked to attempt a six-part version, which defeated him (but resulted in his Musical Offering). What fully went on is shrouded in mystery.

That hasn’t deterred – or rather it has enabled – Cotton, who imagines the humiliated Bach, too old to care, daring to tell truth to power, unleashing ill-disguised fury at Frederick’s laissez-faire attitude to military depravity. He is morally and creatively fuelled by faith, unfashionable as it is, and pits himself against chilly realpolitik.

After a slow-burn set-up, it’s a blazing scene that plays to Cox’s strengths as a stage animal, forbidding as he stoutly stands his ground and locks eyes with Stephen Hagan’s haughty Frederick, but also invested with deep humanity. Cotton embellishes his theme – Bach becoming akin to a challenging but consoling surrogate father to Frederick, whose own dad was a monster – in a scene that, tenuously, has the king dropping into his subject’s lowly abode for a chat.

It almost works brilliantly as a play of ideas – and under Trevor Nunn’s fleet direction, the piece attains a sense of urgency despite its gilded period trappings (we are now back in an age of autocrats, and expect our artists to challenge the status quo). But the under-written nature of Bach’s exhortative wife (tenderly played by Cox’s real-life spouse Nicole Ansari-Cox) is just one element of a script that could do with more fine-tuning – and it would be desirable to hear more of Bach’s royally exquisite music.

Until Oct 28. Tickets: 01225 448844; theatreroyal.co.uk