Violist Antoine Tamestit didn’t seem like much a soloist at the opening of Berlioz’s Harold in Italy on Tuesday night, in Valery Gergiev and the London Symphony Orchestra’s second performance of this programme at the Barbican. His viola lolling casually at his side, Tamestit gazed pleasantly at the orchestra, nodding along with them from time to time. He seemed almost bafflingly at ease, as if he had just wandered into a room where some friends were playing chamber music. When he started playing, it immediately became clear what he was doing there.

He was right, as well, not to treat his part like a straight concerto solo: Harold in Italy is a “symphony with obbligato viola” rather than a concerto, and while it’s a perplexing distinction to draw, Tamestit’s dramatically sensitive performance made the difference clear. The viola part describes Harold (“Childe Harold”, in Lord Byron’s poem) and his pilgrimage around the Italian countryside, where Berlioz himself had also travelled – and over the course of the piece, Harold observes and joins in with the goings-on around him as often as he influences them himself. It’s a more sociable approach than concertos take, and the soloist needs sufficiently little ego to be able to simply join the orchestra and get lost in the texture at some points, and indeed to sit out most of the finale. Tamestit’s attitude to it was perfect.

And his playing was just as good: he gave a remarkably beautiful sound to his opening lines, and found some astonishing pianissimos within his viola. How remarkably clear he was in the second movement, a kind of brisk slow movement à la Beethoven 7 in which the viola has a huge number of arpeggios to play, very softly, on the bridge of the strings. If the third movement – meant to be a lively dance – was a little too subdued, the finale made amends; perhaps inspired by the conviviality of Tamestit, the LSO found their best form of the evening here.

Before Tamestit’s arrival, the concert had been a different story. The first half featured two more Berlioz pieces, but while deserving points for inventive programming, neither the Benvenuto Cellini overture nor the early cantata The Death of Cleopatra found much of a spark. Berlioz and Gergiev are a good match in many ways: there’s an impulsivity, an unpredictability to both of their manners, which makes for some thrilling moments. But in the (rather long) overture, the sudden shifts seemed arbitrary rather than inspired or bold, and while it bounced along effectively enough, it didn’t seem to go anywhere. Typically, a thrillingly fat silence just before the final few bars had enough dramatic tension in it to make amends – but this wasn’t an inspiring rendition overall.

With mezzo-soprano Karen Cargill as soloist, The Death of Cleopatra fared better, with Gergiev and the orchestra subdued accompanists in this pensive dramatic monologue. Cargill sang with superb diction and a marvellously rich upper register – though the lower sections of this piece often found themselves swallowed up. The orchestra was too much for her, for instance, at the piece’s climax. But the most striking part of this cantata is what follows this: a magical, ghostly coda built above an ambiguous rocking fifth in the double basses, as Cleopatra spells out her final act. Cargill’s powerful portrayal of Cleopatra’s final moments was well served here by some captivating orchestral playing.

World-class moments abounded, then, although this wasn’t the most consistent evening as a whole. Ironically, given the humility of his performance, the hero of the evening was Antoine Tamestit.