Herzog - a soccer fanatic - is one half of the massively successful Swiss firm of architects Herzog and de Meuron. Ten years ago, Jacques and Pierre won the competition to design what became - after a further five years - Tate Modern. At the time they had done some well-regarded little buildings, were known as purist modernists who occasionally dabbled wickedly in surface decoration, but were anything but the global superstars they have since become. Now look at them. They're so in demand, they don't even bother to have a website. And they are getting their own exhibition in Tate Modern's great turbine hall.

They have the attitude that gets them routinely called upon to design everything from a Californian winery to a mid-west art gallery, a huge triangular exhibition centre in Barcelona to a sybaritic Prada superstore in Tokyo. (Jacques is wearing Prada when we meet). They won the Stirling Prize a couple of years back for their Laban dance centre in south-east London, and they've won America's most prestigious and lucrative architecture prize, the Pritzker. Plus there's the football, of course. Herzog, a small, wiry man who ought to be a demon striker, has completed two stadia, one in his native Basel and now this new one - which is for both Bayern Munich and their less illustrious 2nd division colleagues TSV 1860, and will be one of the 12 key venues in next year's World Cup.

Hot on the heels of Munich - nicknamed the "rubber dinghy" by locals - comes a third, their "bird's nest" design for the main 2008 Olympic stadium in Beijing, which is as bony as Munich is squishy. There are all sorts of oblique connections there, since one of the reasons Herzog got to build a new Munich stadium is that the architects of the celebrated original 1972 Munich Olympic stadium, Gunter Behnisch and Frei Otto, refused to let it be altered for the World Cup. Apparently in Germany architects have such a right of veto if their building is deemed important enough. But let's get back to that restaurant ceiling.

For all that Herzog professes great admiration for the fortress-like nature of traditional British stadia - Liverpool's Anfield and Wimbledon's Centre Court are two of the sporting crucibles he cites - he must have trouble, as a man who insists on a proper sit-down lunch every day, finding a civilised place to eat at our soccer shrines. Maybe this is why he has given the vast top-grade restaurant at Munich, looking right out over the pitch, an extraordinary busy gold ceiling of conjoined tubes, like a million differently-sized goblets all fused together. Gold ceilings at a footie match? That, and the fact that the translucent skin of the stadium glows either red, blue or white at night, depending on which team is playing, lifts this place above the ordinary. It comes down to Herzog and de Meuron's fascination with the tactile.

You see it at Tate Modern with its rough oak floors and quasi-industrial heating grilles. And it's there to be found at the new Walker Art Center in Minneapolis, where the external skin is a metallic equivalent of crumpled paper. Although there seems to be a touch of Libeskind angularity about their additions to this performing arts centre, it's quite literally not so clear-cut. More as if it's been kicked around a bit.

Herzog is still very keen on the Tate - he has recently returned to add a new ziggy-zaggy shop at the north entrance behind the root of the chimney. Another ingenious Herzog ceiling there, this time in decon rather than baroque style. He is also designing the gallery's internal expansion into the transformer rooms of the south side. "No other city in Europe has changed so radically as London, and the Tate helped to initiate that process," he says. "Now we are going to make a new piece."

Herzog is a firm believer in internationalism. "I don't believe in 'genius loci' (spirit of place)," he says. "The exchange of information is so rapid today. You cannot not be influenced by what's happening elsewhere." There is certainly no homogenised Herzog and de Meuron style. Every building is different, and their output seems to be getting steadily more outré. The early work was quite reticent. Their designs for the Tate were positively understated compared to most of the ideas of competing architects: wisely they let the huge old power station do most of the talking. Elsewhere, though, they work up quite a sweat when it comes to virtuoso design, as the Walker Art Center - greeted with awe by the American critics when it opened last month - makes clear. There are swirly ceilings and chunky chandeliers and all kinds of different surfaces in there, including waxed Venetian plaster and suede. It's a far cry from the pared-down Swiss aesthetic they were once associated with. Understatement is no longer a quality you associate with Herzog and de Meuron.

I try to get Herzog to admit that his style, his approach, has changed over the past decade, but he'll have none of it. "We are more experienced," he fences. "Unfortunately, we are ten years older." So what if he now has 180 or 200 staff, who have maybe 40 buildings on the go? "I wouldn't be able to work on just one project at a time".

However, he is prepared to come up with a metaphor - intended to show why different stadia have such very different atmospheres - which could equally apply to all of Herzog and de Meuron's disconcertingly varied work. Being Herzog, it's a culinary metaphor. "The same wine poured into different glasses tastes completely different," he offers. Beyond that, he says, it's a matter of human feeling. And this he feels strongly about. With his highly-focused stadia, he deliberately tries to cook up the emotions of the fans. In Minneapolis, he tries to bring together the city and the art centre by what he calls "making a spectacle out of the normal". And this is where you get to the nub of the Herzog world view. "We try to encourage people to be energised. What happens in today's society is that tools are invented to keep people away from emotion. To keep people away from being direct. We say - let it out."