Not many musicals make musical history. Once the template is set – the hoofing, the ballads, the knees-up at the end – you know the score, and instant familiarity beats innovation.

But the 1957 production that unleashed Bernstein’s fizzing West Side Story on an unprepared Broadway audience threw out the old formulas. And it wasn’t just a flashy composer’s triumph: the great strength of West Side Story lay in its totality: a show where all the artistic elements blended to produce something that Broadway audiences genuinely couldn’t classify.

“We didn’t know it was going to be an amazing hit,” says Gerald Freedman, now 80, who worked as an assistant to the director Jerome Robbins on the show before its premiere and has since gone on to revive West Side Story many times. “But we knew it was something special. Something completely out of the ordinary.”

In this respect the major UK celebration of the 50th birthday of West Side Story can tick only so many of the boxes. In a glitzy tribute package, Universal has rerecorded the score, taking advantage of its growing stable of crossover performers, neither strictly pop nor strictly classical. Hayley Westenra, the ethereal-sounding, 19-year-old Kiwi, sings Maria; the swarthy Vittorio Grigolo, an Italian operatic tenor, sings Tony.

The thinking is sound: in a kaleidoscopic score, Bernstein does range magpie-like over musical styles from pop to jazz to opera. And the result doesn’t just require immensely versatile singers, but a pit band that can carry out his exacting demands. Conducting the Royal Liverpool Philharmonic on the new recording, Nick Ingman certainly nails it. “Bernstein came from a classical place, not a show place,” he says. “In fact, at the first band call of the opening Broadway show, the orchestra walked out because it was so horrendously difficult.”

But recognising the score’s jazzy intricacy doesn’t mean reclassifying it in a genre deemed more “worthy” – a trap into which Bernstein himself, colossally egoistic to the end, would eventually fall. It’s one thing for Jamie Bernstein (the composer’s daughter) to rhapsodise to me about how “through composed” the score is, “built out of elements that are used all the way through”. It’s quite another, as her father actually did in the 1980s, to remould the thing as a full-blown opera, in a DG recording featuring Kiri Te Kanawa and a preposterously unAmerican José Carreras as Tony. It sold by the shedload, but many were rightly derisive. Handing over the part of Tony to another Latin tenor, as Universal has done with Grigolo, looks worryingly like history repeating itself.

West Side Story worked because it was a team effort – and because the team successfully deflected Bernstein’s highbrow inclinations. It’s true that when the choreographer Jerome Robbins first proposed the idea of a modern adaptation of Romeo and Juliet, relocated to the tenements of Manhattan, and featuring Jewish and Italian immigrants rather than American and Puerto Rican (amusingly, “East Side Story” was the working title) Bernstein at first saw it as an opportunity for “the great American opera”.

He changed his tune once Arthur Laurents, engaged to write the book for the new musical, stepped in. “I want to make one thing clear,” he said. “I’m not writing any f****** libretto for any goddamned Bernstein opera.” The piece would, he pronounced, be “a musical that tells a tragic story in musical-comedy terms”.

Even here, he was only half right. As a Broadway stage show, West Side Story did follow a few fundamentals of the Gershwin or the Rodgers and Hammerstein mould. But with Robbins’s rigid insistence on building the dance throughout the music and the drama, this was a production that took on the challenge of Laurents’s gritty scenario and met it head-on. “No longer did you just bring on the dancers for set-pieces” says Freedman, “the leading roles also had to do the dancing.” It was an enormous innovation that bound together the two disciplines of dancing and singing for the first time.

But the creative tension never abated. Brought on to fill out Bernstein’s awkward lyrics, a young Stephen Sondheim did his best to ramp up their irony and tension, though he later reported that some of them (notably I Feel Pretty) “still make my blood curdle”.

The new quartet – Bernstein, Robbins, Laurents and Sondheim – nipped, tucked and argued constantly. During rehearals, there was as much gang warfare as there was on stage. Once, thinking Bernstein wasn’t in the room, Robbins attacked his orchestration, shouting, “Take that Hollywood s*** out!”, only for the irate composer to storm out to the nearest bar. It can’t have helped that Robbins was conducting affairs with two cast members – one male, one female – during his unprecedented eight weeks of rehearsal.

“Yes, they had differences,” chuckles Freedman, “how much should be sung, how much danced, but it wasn’t ego driven, it was content driven – that was what was wonderful.”

But what comes across most strongly from the reminiscences of both Freedman – and the very first Anita, the Broadway veteran Chita Rivera – is the dominance of Robbins’s vision. Rivera recalls the intensity of Robbins’s direction. “It came from dialogue, it came from the story, it wasn’t ever just steps. Always with Jerry [Robbins], even with no plot, the dance is about something.”

Rivera recalls the hothouse atmosphere that Robbins created in rehearsal. He put a poster up on the cast bulletin board with news of a gang murder near the theatre. “Underneath he wrote: this is your life,” says Rivera. “It was not theatre any more – it was like being hit in the head with a brick.”

What this all meant for the future of West Side Story was problematic. Even by the time of the 1961 film adaptation, which pulled in thousands of new fans, Robbins’s potent brew was being diluted. He himself was fired as director of the film; the end result would later be labelled by Laurents as a “hot fudge sundae kinda film, doing nothing”. It blunted the grit of Laurents’s book, partly by reordering the songs, and partly through the banality of the two lead actors’ performances; their singing was dubbed.

The truth was that unlike any other musical before – and possibly since – this depended as much on the director’s stage images as on Bernstein’s score. When reawakened 50 years on, the result can veer towards the museum piece; in the last major London production, there was carping from critics who wondered why we were getting a “reproduction” production, still credited to Robbins, instead of something refreshed. “But you wouldn’t change Bernstein’s melodies, would you?” asks a sceptical Freedman. “The same is true of the body movement.”

No matter. The West Side Story that remains seems unlikely ever to fade away. Its enduring strength is its blazing, brazen life force. Or, as Rivera says, “It has everything – it makes you laugh, cry and it makes you hope.” It’s a tonic to Bernstein’s own maudlin sentiment, voiced at the end of his life: “You know what makes me really distraught? I am only going to be remembered as the man who wrote West Side Story.” Many would settle for a lot less.

The new recording of West Side Story is out on Aug 13 on UCJ

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