When a celebrity lets their public mask slip, something wonderful and also disconcerting can happen: they can noticeably become someone else. If they’re lucky, that change can be so marked that they become just another face in a crowd.

Cambridge University, cradle of Newton, Keynes and Wittgenstein, of Wordsworth, Turing and Tennyson, has produced 15 prime ministers and more Nobel Prize-winners than most nations. In its 200-year history, the university’s debating society has hosted princes, politicians and leaders in every field: the Dalai Lama, Winston Churchill, Nelson Mandela, and last week a 25-year-old east-London DJ, Kissy Sell Out.

There’s a surreal sitcom waiting to be written about the often-told story of when Charlie Higson and Paul Whitehouse were Stephen Fry and Hugh Laurie’s plasterers for a while in the early 1980s. Here’s the pitch: F and L would play caricatures of themselves in the mould of the posh twits they played in Blackadder, and – for extra comic frisson – H and W would play it straight while appearing (as the story goes) naturally funnier than their professional Oxbridge comedy-writing superiors.

Is there anywhere Stephen Fry will not go? I mean in documentaries. We’ve had Fry on depression and Fry on America, Fry on HIV and Fry on endangered species. Movingly, we’ve had Fry on who he thinks he is, an odyssey in which he discovered that much of his family fetched up in the gas ovens. Fry on Wagner? Admit it, you weren’t surprised. You didn't think, not another bloody comedian investigating, in pursuit of ratings, a subject of which he knows next to nothing. Fry, as everyone knows, knows everything.