If central casting came up with a French superstar architect, it couldn't do better than Jean Nouvel: dressed all in black, with a frame that unashamedly confesses to years of long lunches and a face that, despite his usual geniality, looks perpetually sneery, topped with a shiny dome of a head. In person he plays the part to a T. He speaks every word like a manifesto, as if it were rooted in his heart.

Today's topic for Nouvel's intellectual rhetoric is the difference between the great architect and the great artist. A new exhibition, curated and designed by Nouvel about his late friend, the artist César, opens next week at the Fondation Cartier, in Paris, which Nouvel himself built 14 years ago. “César would come to the openings of my buildings and he'd say: ‘Ah, we artists, we are nothing. When I make something 6m tall I think it so big, but you build 50m high.'

“But I explained, these are not the right parameters. Artists are always the laboratory for the first ideas, the first emotions. Architects take emotions from the streets, the art galleries, the museums; they steal. I have a lot of compromises. César could do exactly what he wanted.” And so he did, crushing cars into neat, colourful oblongs, or making enormous plasticky globules.

Architects often wish they were artists, and Nouvel is of the artier kind. He grew up in medieval Sarlat, in south-west France, the son of school teachers. His parents didn't think much of his ambition to become a painter. They favoured something more practical. Today he has found the perfect compromise. He runs an office of 100, is in his sixties - the most fertile period for an architect - and, at last, he has the recognition of his peers. Last month in Washington he collected the Pritzker Prize, architecture's highest honour.

He is now in a purple patch of commissions: concert halls in Paris and Copenhagen; an outpost of the Louvre in Abu Dhabi; in Qatar a skyscraper and an extension to the National Museum; in New York an apartment tower and, perhaps his most prestigious new commission, the 75-storey Tour de Verre, a soaring, slim pinnacle of a skyscraper beside the Museum of Modern Art. Even architecturally xenophobic Britain has belatedly fallen for his charms, with a massive office complex, One New Change, rising in one of the most sensitive sites in the country, abutting St Paul's Cathedral.

And he has done all this while retaining a conceptual approach that would please any artist. “For me it is the idea,” he booms. “The concept, that is everything. I don't design a lot, or work with models.” He disdains the computer - “it has no emotion, no feeling” - and even the pencil: “I craft with words.” Most of his day is spent debating, describing, cajoling, using words to get across his concepts. He even employs an in-house critic off whom to bounce ideas. “Analysis is everything,” he says. “But at the same time, I don't want to be only an architect of paper. I fear I am a little bit, because I lost so many projects.”

Ah yes. The lost projects, an inevitable consequence of life at the artier end of architecture. In the early Nineties Nouvel lost what's become one of the most famous unbuilt buildings of modern times, the Tour Sans Fin, a cigarette-thin 1,300ft skyscraper in Paris, whose skin graduated from dark granite via steel and aluminium to ethereal glass, so seeming to dissolve into the clouds. Soon after, his firm was bankrupt and he has been paying off the debts ever since. Last month he had final vindication: a commission for the Tour Signal, a skyscraper close to the Sans Fin site, but with a different concept - skygardens - “I cannot repeat!” he cries.

Even when built, though, his work lurches between brilliance and balderdash. The Musée du Quai Branly (2006) was a dog's dinner. There were beautiful fragments, but the whole was cumbersome, heavy and poorly detailed. His extension to the Reina Sofia museum in Madrid (2005) was equally leaden. Even the Pritzker judges snippily noted his “varying degrees of success”, while his close friend Frank Gehry told The New York Times: “He's precarious. He tries things, and not everything works.”

This inconsistency might lose him work, but it often guarantees him glory. He likes to be “an illusionist”, and at his best he exploits the sensory qualities of materials, employing trompes-l'oeuils, and creating magical buildings. “I am proud of this variety in my work,” he told me at Quai Branly's opening. “When it works the architect has made anywhere somewhere.”

César: An Anthology, curated by Jean Nouvel, is at the Fondation Cartier, 261 Boulevard Raspail, Paris (01 42185651), July 8 to Oct 26

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