Romeo and Juliet, Royal Opera House, London<br/>Giselle, Coliseum, London

Two romantic classics look ravishing, but they are also compelling dramas propelled by violence and sheer terror

So, according to the television schedules, we have become a nation of dance enthusiasts. Yet not one of the latest flush of reality programmes features ballet, and no prizes for guessing why.

No first-timer, however game or talented, could make a decent fist of it in a matter of weeks. Watching ballet, though, can be done from a standing start. But where to start? With great music and a gripping story, I say, and just now Britain's two biggest classical companies are offering perfect primers.

Kenneth MacMillan's Romeo and Juliet has rarely been out of the Royal Ballet's planning since its creation in 1965. Far from growing stale, though, it gets regular transfusions of fresh insight from each rising generation of talent. And there has rarely been as good a time to see it. For this run the company is fielding eight Juliets, each of whom brings a distinct sensibility to the young girl's dilemma, and each of whom commands her own formidable gang of admirers. On Wednesday night, I met an Italian who'd come for Alina Cojocaru. She had flown in from Milan late that afternoon, and would be back at her desk next morning.

Cojocaru, being tiny, has no difficulty playing a teen. In fact, when she makes her entrance at the Capulet ball, she could pass for a spoilt 11-year-old, accustomed to being cooed over by her parents' powerful friends but not used to being allowed to stay up late. Her lurch into womanhood happens before your eyes. Cojocaru's special quality, though, is to invest otherwise standard ballet steps with free-ranging imagery. Bouréeing around her nurse, her fluttering feet suggest a pet bird, ruffled and excitable, with a heart in its fragile ribcage at risk of bursting. It's a fleeting idea – there and gone in half a second – but it's the accumulation of such details that make Cojocaru's Juliet worth spending a night in an airport to experience.

Set against the filigree is the brute swirling mass of Verona street life, and it's this, the hormonal stew of MacMillan's squabbling whores and hoi polloi, that powers the story so unstoppably. The fights in this production have sharpened into one of the wonders of the West End stage: fast, athletic, dangerous, the clash of steel a percussive add-on to Prokofiev's most inspired orchestral writing (which itself gets treated royally by Russian conductor Boris Gruzin).

At the centre of this clamorous vortex is the hush of the balcony duet, and I've not seen one more believable than Cojocaru and Johan Kobborg's. A couple in real life, what they present, soapy as it sounds, is total empathy, he alert to her every impulse. For a split second, he can't quite believe his luck when, standing dumbly below the balcony together before the fireworks start, she grabs his hand to feel her heartbeat. Yes, this Capulet girl is fast. It's helter-skelter from then on.

Giselle is another box-office staple that owes its endurance to great dramatic architecture. Again, a tender love story is buoyed along by stagefuls of pinny-shaking yokels, but then the cosiness is swept away by destructive force. "She's not dead, is she?" said my companion, incredulous after Giselle's collapse. But the blanched, undead and, yes, terrifying second half of this ballet is the crux of it.

There are other good productions of Giselle, but English National Ballet's, assembled by the ballet historian Mary Skeaping in 1961, is special. For a start, it restores some of the conversational mime that others have jettisoned and, delivered as clearly as it is by Jane Haworth, Giselle's superstitious mother, it's a joy. Better still, though, its command of the various frissons of fear unleashed by the chorus of Wilis makes for an unforgettable Act II. Veiled in white like young Miss Havishams, these vengeful spirits of jilted brides present a terrifying vision, whether drilled in straight lines or, more insidious still, arranged in fragrant tableaux of 1840s femininity.

In Romeo and Juliet, it's the threat of a deadening marriage that jeopardises life. In Giselle, it's betrayal, assuaged in death. The Wilis, flitting through forests at night, lure men with their glimmering beauty then force them to dance to death. It's a brilliant ruse for choreography: extremes of virtuosity from the leading men (stallion-flanked Fabian Reimair is superb) and the ultimate challenge for a ballerina, to move from the pink of life to bloodless spectre. Fernanda Oliveira's Giselle, so coy at first, ultimately triumphs in sadness. The true stars, however, are ENB's Wilis, a uniform scourge of gauzy girls with the softest of limbs but stone in their hearts.

'Romeo and Juliet': (020-7304 4000) to 16 Mar; 'Giselle': Theatre Royal,Glasgow (0870 060 6647) 17-20 Mar; New Victoria,Woking (0870 060 6645) 6-9 May

Next week:

Jenny Gilbert is dipping into the brantub that is the London International Mime Festival for a show that features a bearded lady