Marriage is a managed retreat from ideas of pure independence and self-expression

"The casting was ready made," said one critic. "Verdi's perfect couple," read the headline over another paper's notice. The work under discussion was the Royal Opera House's La Traviata, a production which offered feature writers and reviewers an extra frisson besides Verdi's musical climaxes, because Angela Gheorghiu, who sang the role of Violetta, and Roberto Alagna, who took the part of Alfredo, her callow young lover, are married in real life. For several writers this clearly conferred on the performances an extra stamp of authenticity. "He sings radiantly," wrote one critic of Alagna, "finding, understandably enough, a natural bond with Gheorghiu as they express their mutual devotion." Well, you often find what you look for in art, so it's possible that a sentimental glow coloured his vision, a more intellectual version of the involuntary coo old ladies give when they pass a wedding (though it's only fair to point out that his judgement was shared by others). It's also possible that he doesn't actually know what it is like to be married, opera reviewing not being a famously uxorious profession.

In which case he could be forgiven for not grasping that a real-life marriage might be a hindrance to the depiction of a fictional infatuation rather than a help. If this sounds glumly disenchanted, it isn't intended to (I am married and I like my kneecaps where they are). But the truth is that marriage and romance agreed to an amicable separation a long ago. They meet up from time to time, naturally - sometimes they even bump into each other unexpectedly, and let themselves get carried away. But they find it virtually impossible to keep house together, with its modest tasks of maintenance, the dull husbandry of everyday life. What a good marriage offers - emotional shelter, a durable, crafted love - generally has to be paid for in the currency of romantic passion - intoxication, obsession, breathlessness, the marvel that you should have met.

Now, it might be that two busy performing artists - often working in different continents and meeting only when their schedules permit - can preserve the latter qualities in a marriage for longer than couples who sleep every night under the same roof (though the history of celebrity marriages actually suggests otherwise - that absence makes the heart grow wayward). But it may be a fallacy anyway that real love can inform a performed love, the elaborate dance of gesture and look by which passion is telegraphed to an audience. Going home with your co-star only adds another weight to the peculiar burden of feeling that actors must drag behind them, their professional requirement to experience the same emotion at the same moment, night after night.

When real relationships inform fictional ones it is likely to be much more accidental or contradictory than a marriage can afford. The smouldering connection between Humphrey Bogart and Lauren Bacall in To Have and Have Not draws some of its force from the fact that the couple fell in love during the filming - it looks as if they can barely keep their hands off each other because, by and large, that was true at the time. And while affection helps in this respect, it isn't exactly indispensable - the feral, dangerous lovemaking of Oliver Reed and Glenda Jackson in Ken Russell's Women in Love gains something from the fact that the actors soon discovered that they were in the grip of a passionate and reciprocated repulsion. Loathing can look oddly like lust, if the context is right.

When marriage genuinely works for actors, on the other hand, the virtues that you see on screen are unlikely to be those of young love or infatuation. In the film of Edward Albee's Who's Afraid of Virginia Woolf? Richard Burton and Elizabeth Taylor's performances are surely informed by the scars of real combat. For viewers at the time, aware that Taylor's marriages were somewhat fragile constructions, there was the titillating thrill of possibility; for later viewers those hellish scenes can be taken as an on-screen rehearsal for a performance that was to take place in private. But bickering can also take more benign forms; the long partnership of Spencer Tracy and Katherine Hepburn delivers a good example of performances which acknowledge the truth of a long relationship - that it cements itself by what can appear rather dyspeptic to outsiders, that marriage is a managed retreat from ideas of pure independence or pure self-expression. Their mastery of companionable skirmishing - which implies some shared wounds and willing surrenders - is a far better model for what marriage delivers to a performance.