It is one of the peculiarities of large scale ballet that as many as half a dozen dancers can share a role within a single run. This may be a necessity given that certain works are physically too shattering to perform every night. And the rotating system also allows artists to encounter a maximum variety of roles within their short careers. For audiences, though, it turns ticket buying into a lottery because changes of cast can so profoundly alter a work from one performance to the next.
Anyone who saw Edward Watson, for instance, tackling the lead in Antony Tudor's Shadowplay last week would have seen a very different ballet from that danced by first cast Carlos Acosta.
Physically the two men are opposites, and this alone impacts crucially on how the movement looks and feels. Acosta is solid and dark while Watson is lean and milkily pale-skinned, and the latter's body actually inserts itself more naturally into the fluid, detailed lines of Tudor's choreography.
The Boy with Matted Hair was first created for Anthony Dowell and Watson possesses a similar capacity to stretch and soften his moves with liquid effect. The clean length of his legs and the refined flexibility of his feet gives his dancing an exquisite elasticity embellished by the luxuriant twist of his torso and the wide carriage of his arms.
Dramatically the two men also play the role differently. The Boy with Matted Hair comes over as a cross between Mowgli and Buddha as he embarks on a bewildering rite of passage to adult enlightenment. While Acosta's Boy is a feral, commanding, almost symbolic figure, Watson's is vulnerable and very human. As the gods of the jungle crowd in on him his huge dark eyes flare with alarm and curiosity and his body twitches with reflex intimations of danger.
It is ultimately a far more affecting interpretation than Acosta's though it is marked by Watson's inexperience in principal roles. His technique lacks the fabulously centred strength of Acosta's and his partnering skills need honing. For those of us who have been tracking Watson's rise through the ranks, though, it's a performance that confirms his star potential.
When Tamara Rojo joined the Royal from English National Ballet this season her starriness was already established - but it had not been tested in a wide range of roles. And dancing Ondine on Thursday - Ashton's flawed but magical fantasy - Rojo's tiny, intelligent body found a sympathetic, testing showcase. Rojo cast herself headlong into the speed of Ondine's capricious steps and gestures, and into the extravagant, playful curves of her body.
While Sarah Wildor's Ondine last week was a creature from another world, Rojo's was an emotional girl. From the giddy thrill of falling in love to the broken anguish of losing her beloved, Rojo played the role not like an alien water sprite but like Juliet reliving her tragic affair with Romeo.
• Ondine is in repertory until December 11. Box office: 0207-304 4000
