Nine assassins? It makes Sweeney Todd sound positively restrained. But even leaving aside the sheer number of killers and potential killers on stage, surely assassination itself is a peculiarly gory and distressing subject for a musical?
'I think there are some people who feel that to treat this material at all in a musical form is to trivialise it,' concedes Weidman, to the muffled backdrop of gunshots on the stage. 'But I think that says more about people's expectations of a musical than it does about this show. If one were to write a straight play about these people, I don't think anyone would feel scandalised. But you put a band between the audience and the play, and some people immediately feel that the material is not being taken seriously. And there could not be more serious material than this.'
The West End has recently welcomed a large, glossy musical about imprisonment in a South American jail (Kiss of the Spider Woman) and a thriller-style play about the trauma left by torture (Death and the Maiden). Yet staging a show based on real and highly emotive events could still invite charges of sensationalism. Weidman points out that he and Sondheim intended to try to grapple with what lay behind the assassins' motives, rather than detail their acts.
'We realised that what we wanted to write about was the USA. It intrigued us that these characters are generally dismissed as being somehow outside the American experience, and are treated as nuts who have dropped in on us and attacked our Presidents. In fact, if you take them and examine them as a group, ideas about who they were and where they came from begin to emerge. And it was those ideas about them as a group that really became the heart of the show.'
Weidman sees links between the vastly different worlds and minds of the nine characters under scrutiny: John Wilkes Booth (the actor who assassinated Abraham Lincoln); Charles Guiteau (who killed James Garfield); Leon Czolgosz (who assassinated William McKinley); Giuseppe Zangara (who tried to kill Franklin D Roosevelt, but missed and shot the Mayor of Chicago instead); Samuel Byck (who tried to hijack a jet and crash it into Richard Nixon's White House); Sara Jane Moore (who tried to kill Gerald Ford); Lynette 'Squeaky' Fromme (the ex-Manson Family member who also tried to assassinate Ford); John Hinckley Jr (who shot Ronald Reagan outside the Washington Hilton) and Lee Harvey Oswald (who needs no introduction).
'If they were sitting round the room here, each of them would articulate a very different reason for why he or she did what they did,' he says. 'But at bottom, there are things in America which we tell ourselves about ourselves. There are cherished national myths which we like to repeat and which create expectations in people which, more often than not, remain unsatisfied. I think people really are encouraged in the US to believe in the American Dream and to believe that if their particular version of it doesn't come true, someone or something is to blame. That's really at the heart of what these people have in common: however varied and different they were, they all have some sense of frustration and disappointment that turns to anger as a result of not having whatever they felt they were entitled to and had been promised them by America.'
When researching the lives of the real assassins, Weidman was surprised by the nature of the existing work about them.
'There has been a reluctance to look at them in a scholarly way. I don't know if it's because it is reassuring to characterise them as lunatics who are not worth examining - because examining the behaviour of an individual lunatic doesn't help you work out how to protect your President. The ones who interested me most were those who most clearly emerged as perversions of certain kinds of American archetype. Charles Guiteau was an American type who, in another context, would have been admired. He was an entrepreneur: he was determined to succeed in the business world, and he tried any number of different enterprises. When one failed, he would pick himself up and try another one - until finally he had tried so many there seemed to be few left to take a crack at, except being an assassin.
'One thing that is astonishing is that for the most part they were peaceable figures - none of them was a criminal. The act of assassination was a punctuation mark at the end of a life which, without that mark, would have been completely forgettable.'
If this sounds like an apology for people who, for whatever reason, did reach for a gun and aim it at a President, Weidman stresses that underpinning the show is a basic horror at the act.
'One of the things I discovered when I was working on Assassins was that my interest in the material referred back to the enormous pain which I experienced when John Kennedy was shot. I can't even talk about it now without becoming upset. It was my first experience of loss. And it is because of the extraordinary grief and pain that these individuals and their acts produced in America that they're worth writing about. And although the show has a variety of different tones, some of which are comic, we have not lost sight of the material's essential seriousness.'
Assassins is a small-scale, revue musical - more a descendant of the Brecht / Weill branch of the family, than of the spectacular, sung-through branch. Weidman and Sondheim (who worked together on Pacific Overtures) wrote in tandem, with Sondheim basing his music and lyrics on traditional American song styles. None the less, the dark, sardonic nature of the show and its material did not endear it to off-Broadway audiences when it opened in New York last year.
'We had very mixed reviews and some of the negative ones were almost hysterical,' says Weidman. 'We had the misfortune to open the week the Gulf war broke out - and there was one negative review which finished up: 'You would think that in the week when war broke out in the Middle East they would have the good taste to close this show.' It was not good timing. I hope that the show's point of view will be clearer here . . . It is material well worth tackling because it raises so many questions about America.'
'Assassins', directed by Sam Mendes, is previewing and opens tomorrow night at the Donmar Warehouse, Earlham St, London WC2 (071-867 1150).