You wouldn't want to piss him off. Say, for example, by asking if there's a right-wing friendly, anti-government message of individual achievement at the heart of his HBO TV series Treme. ''I think that's the most juvenile thing I've ever heard,'' he says, bluntly.

Simon has always written about the rusting over of the American dream. Failed social institutions, public sector corruption, the impotence of reform: these were the themes of his masterpiece The Wire, hailed as one of the all-time greatest TV dramas for its novelistic depiction of the US's failure and fall.

In his latest work, Treme, a jazz and food-drenched plunge into New Orleans after hurricane Katrina, Simon wanted to show something as simple as a brass band could rebuild a community. ''Culture is what worked, was what brought the city back,'' he says.

But he is also playing a Wire-like long game, one he says will be fully revealed only if he can persuade HBO to give him a fourth season after the third, which has just wrapped.

New Orleans is the US. The hurricane is the financial crisis. And the lack of preparation for either was more than an accident of history - it exposed a flaw at the heart of the US. ''A lot of the things New Orleans presumed to be true in 2005 - about the levees that were protecting their city, about their infrastructure, about their place in the American firmament as a major city - a lot of that got washed away by the flood,'' Simon says.

''They were left with the realisation that they were kind of on their own, that all of the infrastructure that was there, that presumably made things plausible and gave them some sense of their place, it was all spit and chewing gum.''

Three years later, a financial hurricane left households throughout the country underwater figuratively. ''The flood control didn't quite work there in the same way,'' Simon says.

''For me, that's been the political meaning of [Treme]: what's real and what you can rely on in the end.

''New Orleans, to the extent that it has come back, has proved that community and culture are the only two reliable things in American society right now.''

Simon has long been fascinated with New Orleans. He and co-writer Eric Overmyer (a bespectacled, professorial counterweight to Simon's intensity) saw in Katrina's aftermath a chance not only to celebrate its unique people, music and food, but also to diagnose the American disease. ''Pay attention to the things that the characters are trying to reform,'' Simon says in response to the libertarian accusation, listing the (rorted) housing policy, the (corrupt, violent) police department, the (impotent) state's attorneys office, the (crippled) school system.

''The things they are contending with are the things that you need to have a functional modern society. You can't walk away from these things, you can't absolve yourself of responsibility for citizenship. [In post-Katrina New Orleans] there was such a lack of political leadership and a lack of economic clarity and economic will. But to mistake that for the notion of 'there is no society and we all do it ourselves' is …''

Yes, it's the most juvenile thing he's ever heard.

Season three of Treme is not all politics. It revels in the music, packing in more cameos from revered musicians than before, and it lingers over dinner tables in New York. It also introduces a new character, a journalist based in everything but name on A.C. Thompson, who exposed the case of Henry Glover, a victim of a shocking police shooting and cover-up in the wake of Katrina.

Simon says New Orleanians have come to embrace Treme for showing the city beyond French Quarter sleaze and trolley-car cliches. But he is not afraid to ruffle feathers. One sheriff even blacklisted the show from filming in some locations, because he was offended by a fictional character's story.

''I'm going to criticise what's fair to criticise,'' Simon says. ''I don't have carte blanche to write. But … at a certain point truth is a defence.''

Wendell Pierce, a New Orleans native who starred in The Wire, was one of the first to be cast in Treme, as trombonist and wandering soul Antoine Batiste. Pierce's family neighbourhood was submerged by the Katrina floods and is one of the slowest to recover. Pierce has returned to make New Orleans his home, taking personal responsibility for efforts to rebuild - so he has been living the show even while making it.

''When you lose your city that so identifies you, it forces you to commit even more to it,'' he says. ''The idea of New Orleans being spoken of in the past tense was unacceptable to me.''

But even as he rallied his community to rebuild, Pierce came up against some of the problems depicted in Treme. He is still waiting for federal grant money to lift houses above flood level - it is tied up in city red tape that he believes was put there to divert millions into the hands of political cronies.

''In times of crisis, you see the best of people and you see the worst of people,'' he says. ''My project is dying on the vine.''

Pierce says Treme has become a kind of ''group therapy'' for the city, with viewing parties every week. ''People are reminded, 'Wow, that's what I went through three years ago, yeah, I remember that time.' It celebrates our triumphs and deals with our failures.

''It shows you how we are bringing back our city with our culture, from the grassroots up, and saying this is important, this is so important, these little personal, individual battles that collectively make sure that the city comes back.

''It's not the greatness of a visionary, some sort of Marshall Plan that's happening; it's really street by street, house by house, block by block, neighbourhood by neighbourhood, the resolve of the people in New Orleans showing that humanity is the thing that connects it to the world.''

Treme

 Showcase, Mondays, 8.30pm