Happy the young musician with a film-maker for a friend. At least if the film-maker is as talented – yet apparently blessed with spare time – as Martin Mirabel. Close up and personal, here is the Lucas Debargue I remember from spending time with him in Verbier three years ago. Mirabel had already started work on this revealing portrait, but the Alps don’t figure in it. With the two of them we criss-cross countries and continents, beginning and ending in Moscow, scene of his popular success in the 2015 Tchaikovsky Competition.

Fourth place reflected the polish and most of all the formal training of his fellow competitors, but where are they now? One of them I recently saw in Sochi plying his trade in a Beethoven marathon for ‘promising artists’, whereas Debargue flew out of Moscow with a Sony contract in his back pocket, as well as this film in the making. Not that success has toughened him up, it seems: he likes to walk around an empty hall before a concert and visualise the audience arriving, ‘about to see me fail’. This is the kind of honesty that comes from talking not to a camera but a friend.

Indeed, Debargue pays tribute to Mirabel – ‘Martin made me play again’ – along with his teacher, the ever-present Rena Shereshevskaya, and so candidly that it cuts through the schmaltz. They first met as very young men and spent nights drinking, smoking and listening to music – and evidently still do, though now in Chicago jazz clubs instead of student digs. Debargue complains (as he did to me) about the solitary mechanics of a concerto engagement; about adjusting to the fame, the selfies, the obligation to wear a mask and play a role, the struggle to keep his performances alive and dangerous in the face of a full diary of engagements.

Then we hear him play, especially with the Castro-Balbi brothers as a piano trio, and the tetchiness melts away. More of them, and more of him making music than the slow movement of Medtner’s Op 5 Sonata and a Duke Ellington improvisation, would have been welcome for those of us who value the simplicity of his touch at the keyboard as much as his questioning musical mind (try his Scarlatti, 11/19). Perhaps for want of illustration, the film skates over Debargue’s difficult years, spent not at the piano but stacking supermarket shelves. In other respects, however, it spares him nothing, pursuing him even into the sanctity of the rehearsal studio, where Shereshevskaya tears a strip off his Grande valse brillante (‘Your sustain pedal is boring. Let it go!’). Few enough young musicians of renown have the modesty to admit that they need the music more than the music needs them. Will he retain it? If Mirabel returns in a decade’s time, we’ll find out.