Invited to consider the suggestion that the audience might have recoiled from the play's content, which includes descriptions of masturbation, paedophilia and sex with an "enormous woman" who lets out "elephant moans", he responds cheerfully: "Ah, but I thought it was well written! When I was first exposed to the films of Ingmar Bergman, I found them frank and disturbing portraits of the world we live in, but that was not something that displeased me. They were beautiful. I thought people would respond to my plays the way I responded to Bergman's films."

In the mid-1980s, the sexual vaudeville of Shawn's early work gave way to political concerns, though the dramatic procedure was the same: just as the lust-filled reveries that occupy a person's private thoughts remain hidden from the world at large, so, in the second act of Shawn's playwriting career, the obscene oppression of the world's poor by rich and powerful nations is covered up by hypocrisy and, if necessary, military force

The original production of 1990 was described by Frank Rich, then the resident theatre critic of the New York Times, as "nothing if not a musty, radical chic stunt destined to be parodied". A revival in 2007 prompted the headline in the same paper, "The World's a Mess, and It's All Your Fault". Rich deployed the weapon that is used frequently in attacks on Shawn's plays, calling The Fever "an almost entirely humourless assault on the privileged class by one of its card-carrying members".

Shawn, who was educated at private schools before going to Harvard and Oxford, posits a defence based on sincerity. "Being in poor countries was something that changed me more than anything else." He wrote The Fever after visits to Central American states where ruthless dictatorships relied on backing from the US government. "It gave me an insight into the sickness of my own society." The epiphany gave him permission "to devote my life to being a writer". He has an engaging belief in the transforming power of literature.

In Hare's view, the reconstruction of Shawn as a political playwright happened as a result of his exposure to British theatre. "Wally came to Britain in the 1970s, and fell under the influence of Howard Brenton, Caryl Churchill and myself. Until then, the main tendency of his work had been surrealist."

When A Thought in Three Parts, which incorporates the comic youth hostel orgy, was mounted by Joint Stock at the ICA in 1977, it led to protests from members of parliament and a demand on the part of the Conservative MP for Staffordshire South West for "urgent talks with the Arts Council". There exists a theatrical legend that the play was banned; the reality, though less sensational, is equally dismal. Threatened with the loss of its grant, as Hare recalls, the ICA "decided not to extend the scheduled run, despite sold-out houses".

A note at the head of The Fever says that it was "first performed in various apartments in New York City". The performer was the author. It has since been played in many countries, by actors of both genders.

"The trouble with theatres is that people are sitting in seats that they have sat in before, to watch other plays. They are settling in for a certain type of experience, bringing with them a lot of preconceptions that start filtering into their brains before the play has even begun." In spaces such as an abandoned men's club building, where The Designated Mourner received its first and only New York staging, "the audience doesn't know what to expect, and is perhaps more willing to meet you halfway."

"I have lived in the shadow of my father. It would be ridiculous to pretend that there is no relationship between the sex in my plays and the reticence about sex which my father practised, even carrying it over to his duties as magazine editor. It's almost too obvious to mention. Clearly, I must be in some sort of reaction to that, although" - he does the comic deadpan - "I've taken it awfully far."

Grasses of a Thousand Colours is like Shawn's early plays, in being domestic drama of an outlandish kind. Animals don fancy costumes and welcome humans to a party. Cats make satisfying lovers. Husband and wife take pleasure in savaging one another, just as they do in other works by Shawn. Donkeys stand on the table at dinner and "haphazardly piss", while men admire their penises. "Of course it has upsetting elements," Shawn says, looking as if, as was once said, the strongest four-letter word he could utter is "Gosh". He insists that writing "is not a voluntary process.

Isn't all writing to some extent about trying to get through the layers of propaganda and false interpretations and received ideas and clichés that prevent us from seeing what's going on? I think that's the enterprise."