Welsh National Opera's Christmas card (seasonal greetings from press officers to critics are a big thing these days) presented a very unChristmassy sight: a man in pyjamas, staring straight ahead, sharing a double bed - which appeared to be fixed upright to a wall - with a large skeleton whose skull was lightly covered with a halo of red hair. This was a production pic from the WNO's hit show of last autumn - Tchaikovsky's Queen of Spades - directed by Richard Jones and with designs by John Macfarlane, not that the card mentioned either name. This vertical bed at the back of the set was the setting for the scene where Herman imagines the Countess's funeral and works himself into such a state that he sees an apparition of the Countess and learns the winning three-card trick with which he is obsessed. It was one of the most visually striking episodes of the whole production, and drew a sharp intake of breath and a touch of nervous, anxious laughter from the audience as the bed started to heave and bony fingers crept out from under the duvet. The skeleton evoked, if it did not obviously represent, the old woman whom Herman had, in the previous scene, frightened to death, in his desperate bid to obtain her supposed secret.

You might think skeletons are old hat, the stuff of ghost trains and medical students' rag weeks. But a giant skeleton reading the book of life was the presiding image of Jones's enormously successful production of Verdi's Un ballo in maschera at the 6,800-seater Bregenz Festival floating stage last summer (designed not by Macfarlane, but by another designer with whom Jones often collaborates, Antony McDonald). In both cases, the image performs its ancient role in western iconography, symbolising the vanity of human passions. Why it is so effective largely results, I think, from the violent contrast between the theatrical artificiality of skeletons generally (though there's still a frisson about human remains in murder investigations) and the bubbling emotion and strikingly physical, colourful depiction of passion with which Tchaikovsky's and Verdi's operas are concerned - and which Jones's forensic skill at handling performers distils so potently. The theatre may be serious, but the susceptibility and delight of the public lie in the way theatre and opera can be at one and the same moment outrageously blatant and vulgar, yet also deadly earnest. That, surely, is the secret of theatrical accessibility, and it is also something that the modern media world, with its categorisations, finds impossible to match.

Visualisation is profoundly important in opera - despite what we are always told about audiences being interested only in the music. It is true that, thanks to CDs, the music is increasingly detachable from the totality of the operatic experience in the theatre. In opera, music is genuinely the essence, but design is also a notable and well-recorded part of operatic history from its earliest times. In this context, directors are arrivistes. Opera designers date back to before the arrival of historical costume in the theatre. Today - in the age of the director - designers seldom get proper credit for their contribution (and lighting designers do even worse). Macfarlane, who is well known in the ballet world, ought to be a familiar household name in Britain.

Opera is much like a military operation, involving large numbers of people in numerous minute tasks. A highly collaborative team effort, it is usually all the better for different creative inputs. In the past 20 years in Britain, the theatrical side of British opera has brightened out of all recognition. The controversial names in the headlines have been directors: foreigners such as Lucian Pintilie, Andrei Serban and Harry Kupfer at WNO and the American Peter Sellars at Glyndebourne; or young turks (mostly Brits) at English National Opera (and, occasionally, at Covent Garden). But what has powered this process is the new generation of stage designers - David Fielding, Nigel Lowery, Tom Cairns, Antony McDonald, Anthony Baker, to name but a few. Directors who think they can do it all themselves usually run out of steam. The relationship between directors and designers, the primary creators of productions, is virtually marital when it works.

Not that it always does. Shortly before Christmas, I went to the Theatre Royal de la Monnaie in Brussels - where Covent Garden's incoming music director, Antonio Pappano, is currently working - to see Falstaff, designed by Macfarlane for the German director Willy Decker. The lack of laughter rapidly suggested that something was wrong. Not all German-directed Falstaffs are fun-free. Decker has done original and powerful operatic work in Brussels with Macfarlane (a memorable Peter Grimes in 1994). But whose concept was this Falstaff anyway?

Verdi's opera is about the appetite for life, something for which Belgians have quite a reputation (you eat well in the Euro-capital). But this staging was updated to the age of Agatha Christie and set in what purported to be the Windsor station refreshment room. For those of us used to privatised rail standards, this was bizarre enough. But what a pallid Falstaff was made by the veteran Belgian star Jose van Dam - thin, dry, devoid of affection or enthusiasm. The problem with a single refreshment-room set throughout - however Europeanised with a late 1990s bar on one side of the stage and Italian cafe-chain tables and chairs all over - was that it couldn't represent Falstaff's kingdom, or suggest his stomach, drunkenness or cheating. It's a public space.

This Belgian Falstaff lacks Macfarlane's usual romantic flair - no painterly front-drop cloths with images to comment on the piece and steer the audience's response, for instance, without which no Jones production is complete. Pappano's conducting was lively and idiomatic. But Gwyn Hughes Jones's boorish Fenton couldn't blossom into the luscious adolescent kiss between the young lovers, for which Verdi provides one of the most memorable musical ideas. Nor could the manic search of the "station" by Ford's men make sense. One longed for some Harry Potterish humour and sentiment in the final-act night games, where fairies are supposed to pinch and punish Falstaff, and Ford is powerless to stop the nuptials of Fenton and Nannetta (Patrizia Biccere). But the only visual suggestiveness was a twiggy tree pushing in through a vast window, recalling Herne's Oak in Windsor Park. Here was a production handicapped by its scenic concept. Pappano is keen on Decker's work. Covent Garden will be getting Decker after the new regime starts in 2002. Fingers crossed.

Yet Macfarlane's Falstaff isn't a sign of the designer's imaginative decline. It was created for Florence's Teatro Comunale in 1998, a few weeks after his brilliant Hansel and Gretel for WNO opened, with Jones directing. Macfarlane, with Jones as director, has scored two consecutive mega-hits for WNO in recent years, both conducted by the remarkable new Glyndebourne music director, Vladimir Jurowski. Like Pappano, Jurowski is a musician seriously committed to the theatrical side of opera. It is interesting that, after a very difficult period for opera in Britain, and with Paul Daniel and Nicholas Payne already proving their mettle at English National Opera, the three major opera institutions in Britain find themselves with promising leadership, in every case strong on the theatrical side.

For WNO's Queen of Spades, Macfarlane's opening set was very unflashy - just three park benches, ranged diagonally across the stage, beside which kid soldiers paraded, nannies pushed their prams, and officers in mufti introduced us leeringly to the obsessive outsider Herman (an extraordinary, shifty, possessed impersonation by the distinctive Russian tenor Vitali Taraschenko). Macfarlane's opening front-cloth presented a staring-eyed painting of the Countess in youth, which then eerily peeled down to show her in bloated age. The director, Jones, boosted the significance of Susan Gorton's marvellously fleshed-out Countess. As Liza and the girls sang around the piano on the main part of the stage, the Countess was showing the governess a nostalgic photo album in a sewing room next door (Macfarlane and Jones updated the setting to a bit later than 1890, when the work was composed).

Macfarlane turned the pastoral in the middle act into an ingenious, evocative puppet play. The servants leading the Countess to her bed (or hip bath, as WNO's production had it) brazenly revealed their boredom with her tales of past glory, making eyes at each other and at the audience, to show they had heard it all before. Realism flowed in and out of surrealism - typical of Macfarlane's and Jones's work. Yet the success or failure of opera productions is almost never attributed to designers. The director is where the buck stops, exercising ultimate control, but designers are often where the ideas originate. Jones in Bregenz, and also with his Pelleas and Melisande for Opera North and ENO, shared the credit, for both directing and designing, with his designer, Antony McDonald.

To understand the visual power that live theatre and opera can have, one needs the chance to see this sort of work. But all too often, such brilliant examples in the field of design and production are made available only in a few places for a short time. That is one of the problems with having very few opera companies in the UK and severely limited funding for touring. The tragedy is that, despite the extraordinary work there has been on British opera stages in recent years, if you missed it, you will never get the chance to catch it again. And it's not the same on DVD.

The Faber Book of Opera, edited by Tom Sutcliffe, is available in bookshops, priced £20