On the other hand, the professional life of a "concept" designer tends to be short. They ride in on fashion, and fashion's wheel throws them out again: adapt or die. It's time this week to salute the longest-serving professional in the business, as he pulls a rare trick: simultaneously reviving A Christmas Carol at the Barbican, and launching a dazzling La Belle Vivette at the Coliseum. Cue John Gunter, desperately sprinting between locations, and protesting that he's just a jobbing craftsman, nothing to do with "art".
It is a shade modest. He is the man who, for The Rivals, filled the Olivier with giant segments of an 18th-century terrace rotating in a stately minuet, and whose neon-crazy designs for Guys and Dolls are to be recreated there next year. He did the RSC's gritty Maydays and its gaudy Mephisto. His Glyndebourne La Traviata and Marriage of Figaro were televised. He's designed in both Americas, and all over Europe. His work is too varied to pigeonhole, but it's always exhilaratingly extreme.
He started out as high priest of social realism at the Royal Court, presiding over Bond's scandalous Saved. He became the favourite British maverick of the Austrians and Swiss, until the burghers of Zurich decided they'd had enough of his provocations. He then ran the theatre department at Central. "But I found you can't teach design. You either have it, or you don't."
As "director's theatre" rose higher in the firmament, Gunter grew to loathe it, a reaction spurred by guilt. At Bernard Haitink's anguished request, he had to find a way of preventing the audience applauding every time the curtain rose on his sets for the Glyndebourne Falstaff. His design for the National's Government Inspector - a sea of paper cascading on to the stage, with a giant superimposed moving image of the Tsar belching - had been wonderfully effective in establishing a mood of small-town paranoia. "But it was too effective: it pre-empted the play. It took the actors 20 minutes to get the audience's attention. A great mistake on my part."
Gunter was invited to a meeting at which actors put designers in the dock and asked them to account for themselves. "They were sick of sets which either swamped the play, or were unactable upon, or were just plain dangerous. It's not generally realised what a dangerous place the stage is."
Singers in particular seem prone to injury. Gunter refuses to be drawn on who, or where - these things are hushed up not only by the victims, but by opera houses terrified of acquiring the wrong sort of reputation - but it appears that a number of major singers have recently injured their backs or broken limbs at work. With rehearsals taking place on half- built stages - as happened last week at the Coliseum, where rapidly turning revolves added to the hazards - the whole thing is fraught with danger. "You go on stage and look up, and see anything up to 15 tons of stuff hanging over your head: the sensible thing is to get out of the way. It's ironical that whenever a new set goes up, the stage hands have to wear hard hats - but the singers don't."
Designer hubris, moreover, has provoked a backlash. "Leading conductors now often demand to vet your design before you're hired. They want to be able to see that the singers they're conducting are safe. Riccardo Muti vetted me in Milan, and Dohnanyi did it to me over the Flying Dutchman at Covent Garden. Though in that case I think he would have been right if he'd said no." A boat rising from the floor while its spars shot from metal walls; a huge unsupported staircase emerging from the wings; a turntable whizzing round, and constant mechanical foul-ups. "The singers had to look out for their lives. It made us all sick with worry. We were stupid to do it."
The most significant change he's made to the revived A Christmas Carol won't be noticed by the audience: a "limit switch" by which any truck moving across the stage - Scrooge's office, or Bob Cratchit's kitchen - "sees" an obstacle and automatically stops, rather than rolling on over it. When that obstacle is a human body, the advantage is clear.
But computer-controlled scenery - a state-of-the-art fetish in some theatres - is a different matter. "When it works well, it's amazing. You can choreograph movement for five-ton constructions, and make it fit the music. But if it goes wrong, it's hard to override. For example, we use stage-hands in the wings with ropes in Don Quixote, when Richard Van Allan flies up on the sail of the windmill. If things went wrong and we were on a computer, we might have to push a button and leave him hanging upside down. It's better to have the human element, to respond to a dangerous situation."
There's a lot of heavy clobber flying around in A Christmas Carol - a suspended forest of houses which gathers to create a Dore-like cityscape, and disperses to the winds when Scrooge takes flight across the sea. The carolling in this popular extravaganza may sometimes hold up the action, but it's a charming show, very much in the spirit of the original story.
Extraordinarily, A Christmas Carol and La Belle Vivette also share the same director - Ian Judge, with whom Gunter now forms one of the theatre's most enduring creative teams. Who imposes his ideas on whom? It seems they talk a lot, then Gunter goes away and sketches. He doesn't pore over sourcebooks - he didn't use Dore for Don Quixote; he prefers to let his imagination work untrammelled. Then, as early as possible in the process, he makes three-dimensional models.
Models past, present, and future line the walls of the workshop of the house in Muswell Hill where this rumpled, tenacious man lives with his dancer wife and photographer children. The combined effect of his models jumbled together with the tin toys he has collected all over the world is exuberantly spooky.
He goes round the room pointing out highs and lows of his career. "A wonderfully ill-fated White Devil," with Glenda Jackson, Jonathan Pryce, and a hopeless director whose name he forgets. A very pretty set - a lone woman and a huge tree - for a show that never got made. White verandahs for Porgy and Bess; a cash-register filling the entire stage for an Austrian variant on Volpone.
One of the most charming models has a blue eagle spreading its wings across the outside of the proscenium arch, and a matching blue interior. This was for a pacifist play in which the hypocrisy of Swiss attitudes to armaments was lampooned. The back of the stage had been designed to open and reveal an audience of caricatured Zuricher bigots, but it didn't get that far. "That was the only time in my life I've ever had a design censored."
Monday's premiere of La Belle Vivette opened with an unnerving crash, as a revolving flat fell against a lamp-post which broke in two. The cast and crew steered deftly round what could have been a major disaster and the show swept triumphantly on, with its nude flesh-and-blood statues, and its scenery in a constant crazy waltz. Gunter hasn't gone soft with success: he's still living dangerously.
n 'A Christmas Carol' is at the Barbican, London EC1 (0171-638 8891) to 13 Jan; 'La Belle Vivette' at the ENO, London WC2 (0171-836 0111) to 30 Jan