There is a line in Eric Schlosser’s new play that sends a shiver down the spine: “We should finish this business and finish it in a way that does not dishonour the dead.” Not a comment on the war in Iraq, as you might expect from this most polemical of contemporary historians, who put us off our grub with his grim bestseller Fast Food Nation.

This is the voice of a weary George Washington during the long, hot summer of 1787 when the Founding Fathers met in Philadelphia to thrash out the American Constitution. “But it works for Iraq, and for Vietnam too,” Schlosser agrees enthusiastically. “How do you really honour the dead, by having more die? Or do you just accept the futility of their deaths?”

We are sitting outside Shakespeare’s Globe, where rehearsals are under way for We the People, Schlosser’s drama about the summer in which that phrase was penned. Schlosser, straight out of rehearsals, is hungrily chomping a prepacked sandwich, whose provenance I cannot help but wonder. It is not the only detail that seems faintly off. Why has a writer famous for his investigative journalism penned an historical play? Why is it being staged in London and not his native America? And why on this unlikely stage?

It was the success of Fast Food Nation that brought Schlosser to fame, but long before that it was history and then drama that absorbed him. After two history degrees – including a postgraduate spell at Oxford – he became a screenwriter, but in time discovered, to his frustration, that it was a director’s, not a writer’s medium. “If you care about every word on the page, then you shouldn’t be a screenwriter,” he notes. “And I had zero desire to direct.”

His break into journalism came at The Atlantic Monthly, one of America’s finest outlets for literary journalism, when an editor found his pitch in a pile and liked his writing, if not the idea. Another proposal later – to shadow the NYPD bomb squad – and he had himself a gig. Fast Food Nation followed in 2001.

Meanwhile, Dominic Dromgoole, the British director, had unearthed an early script that Schlosser had written but had yet to be performed.

Americans, set at the turn of the century when the British Empire was on the wane and the American rising to take its place, was staged at the Arcola in 2003. When Dromgoole, now artistic director at the Globe, was planning his current Renaissance and Revolution series, he turned to Schlosser with the idea of bringing to life the writing of the American Constitution. Schlosser leapt at the idea. “The America that exists today is unrecognisable from what the Founding Fathers set out to create,” he says. “I think if they could see it they would be horrified.”

His drama takes us inside the room where ferocious arguments broke out over such issues as maintaining a standing army or the wisdom of enshrining the country’s power in the person of one man. They fizz with contemporary frissons. “The liberties of Rome became the final victims of her military triumphs,” James Madison notes. “A standing army is therefore dangerous – at the same time it may be necessary.”

Edmund Randolph, arguing for the power of impeachment, cautions: “The executive will have enormous opportunities for abusing his power, especially during times of war. We must have the power to remove a corrupt and deceitful man from office.” Perhaps surprisingly, there is no debate on separation of church and state, a daily theme in modern American life. “It simply wasn’t an issue,” Schlosser says.

That British audiences may be ignorant of the details of American history should not be a barrier, he insists. “Most Americans don’t have that history either,” he quips. What Americans do have is a reverence for their Constitution as an almost sacred document. “This drive to revere the Constitution, it’s the same as believing literally in the Bible.

The same people who cite bits of the Bible to condemn homosexuality, you don’t hear them citing the parts that approve of slavery, or approve of polygamy. The same is true of the Constitution. It’s a thing written by men,” he says. “But some of them are a**holes.”

Schlosser’s last dramatic outing was his collaboration on the film Fast Food Nation, which borrowed the book’s themes about migrant workers and food production but threw aside the text. It was not a huge hit. “The film played in theatres in the US for 12 minutes,” he says. “Well, a little longer than that, but it was really dumped by the studio there. I think they must have made a calculation that Americans don’t want to watch poor Mexicans abused and exploited.”

He doubts that an American theatre would have had the courage to take on a project as iconoclastic and expensive as We the People. “When theatre works, I think it has a power that’s so much more immediate than any other form, book or film,” he says. “But when it doesn’t work, of course, it sucks.”

It is hard to tell if that is a fear that haunts him. Schlosser bubbles with enthusiasm and praise for his colleagues, but by his own admission has spent his career doing things he never quite felt qualified for. Fast Food Nation’s success was greater than he could ever have dreamt of and the main product of that success, for him, has been to allow him to concentrate on the work that he loves, the long investigative pieces that fewer and fewer magazines have the time and money for.

He shuns the limelight for himself and loathes celebrity, about which he knows a thing or two. His wife is Shauna Redford, daughter of Robert, who he met growing up in Beverly Hills. He retains the greatest respect for his famous father-in-law. For the rest of Hollywood, less. “From an early age, I learnt first-hand about the hollowness and stupidity of celebrity culture,” he says. “There is not now, and never will be, a shortage of famous idiots.”

Schlosser, Shauna – a painter – and their two teenage children now live in a “quiet, beautiful” part of California, having moved back from New York. His kids are pretty happy about this – the West Coast is home to In-N-Out Burger, the only fast-food burger joint that passes Schlosser’s exacting standards.

“They weren’t too happy when the ‘no more Happy Meals’ edict was issued,” he admits. Fast Food Nation’s author takes care to buy locally and organically, “but I’m not obsessive about food”, he adds. “On my list of fear and anxieties, food isn’t in the top ten thousand. Aside from brains, bladders and a few other nasty things, I’ll pretty much eat it.” So the sandwich passes muster.

Once the play is wrapped, Schlosser will return home to his latest book project – an exploration of the American prison system. It is a subject that caught his imagination ten years ago, when he was researching an article on the war on drugs and came across a marijuana dealer in prison for life. Soon after, a woman he knew was murdered by her own boyfriend. He was jailed for two years. “It made me start thinking about who is in prison, and why they are in prison,” he explains. “In the past 30 years we’ve gone from 200,000 people in prison to 2.2 million. And no society has ever locked up that many people because no society has ever been rich enough to do it.” Expect another searing and forensic indictment – but prepare to be moved as well.

No wonder, perhaps, that this theatrical outing comes as such a pleasant interlude. “It’s a real pleasure to make things up,” he laughs. “Most of what I write journalistically is so intense, so fact-based. There are so many companies and people who want to sue me; the whole thing is so dense with footnotes. This is a release.”

— We The People is at Shakespeare’s Globe, London SE1, from September 2 until October 6 (020-7401 9919; www.shakespeares-globe.org)

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