For Gordon Newman, however, it is not a question of preferences but of moral absolutes. 'Liberty is not the most important thing we have,' he argues. 'Compassion is the most important thing, and you can't compartmentalise it: you can't go out and kill a pig in the morning, and in the afternoon embrace your children as if nothing had happened. Murder is murder, and the murder of an animal is the same as the murder of a human.'
There are several large - and, I suggest - subjective ideas here. But not for him: 'I don't believe a deep truth is subjective and open to interpretation.' Such certainty might be considered, by some, smug. But there is no question that it is lived-by as well as preached.
Newman has been a vegan for 10 years, and was a vegetarian before that. A novelist, playwright, and frequent television writer, he has never before insisted on particular dietary requirements for anyone working alongside him. But this latest work, The Healer, a two-part drama to be shown on BBC 1 in the autumn, is different, he says, partly because he is the co- producer. 'As a producer I am notionally the provider of the food, and have to take responsibility for that. And then, I couldn't see myself making a film about a healer and in the process destroying life.'
He is a spare man, evidently fit, and stylishly dressed, although his leanness gives his face a skeletal, skull-beneath- the-skin quality somehow at odds with his clothes. His office at the BBC in Cardiff, where the film will be made, is spartan: apart from a couple of bottles of fruit juice and a jar of olive paste on the window-sill, there is little to define him, nothing on the notice-board. He prefers, he says, to remain uncluttered.
He is 48, and lives in a Victorian house in Herefordshire with Rebecca Hall, whom he describes as 'a radical vegan and animal-rights activist', who recently issued a challenge to battery farmers to spend a week in a chicken cage for pounds 10,000. (No one managed it.) They have two grown-up children. Beyond that, he is reluctant to talk about his background. 'That's a rather Daily Mail question,' he says, when I ask where he grew up. 'Kent. And there was nothing in my background which led me to become a writer.'
He rarely smiles, though that may be because he is mistrustful of journalists. (The investigative reporter Duncan Campbell sued him over his television film Here Is The News, claiming a character was based on himself. 'Sadly, it wasn't fought: the BBC settled,' Newman says). Occasionally there is a light in his eyes which warms them; and certainly those who work with him are enthusiastic on the subject of his charm. But there is no denying his capacity to annoy.
Apart from journalists, his targets have included the police (his belief that corruption is endemic in the service was a theme of his first novel, So You Bastard, and his recent television play, Black and Blue) and doctors. The Healer is about a young doctor who discovers that he has healing powers when he touches people - much to his own and his superiors' discomfort.
In The Nation's Health, Newman expressed more directly his conviction that 'in the average general hospital, one- third of people are suffering from illnesses induced by the medical profession'.
He doesn't use doctors himself. 'I haven't been ill for the last 20 years, but if I were, I know they would have no ability to help me.' I tell him about a woman I met recently, who had had bowel cancer, and who is so grateful to her doctors for her extra years of life that she has devoted herself to saving their hospital. 'Was she a vegetarian?' he asks. 'Bowel cancer is the cancer most linked to meat- eating.' He may be right - but so much, I felt at that moment, for compassion.
'If she had been thrown back on herself, without doctors,' he suggests, 'she might have achieved some recognition which would cause the disease to remit.' In fact, she would have been dead.
What if he were to develop cancer? Would he seek orthodox medical treatment? 'I can't even contemplate the notion of getting cancer,' he says. 'It's not in my consciousness.'
He claims there has been 'no progress in the efforts to find cures for cancer, nor ever will be because the research is based on evil, the torture of animals in laboratories. And you cannot have good from evil'. What about child leukaemia, I ask. Children now have a 65 per cent chance of survival, compared with 5 per cent in the early Sixties. 'Nonsense,' he says. 'Absolute nonsense.'
Underlying all this there is, I sense, an idea of a vegetarian moral superiority which is somehow insulating. He argues that killing animals spreads moral contagion: 'Every time you destroy a life, you are putting violence into the atmosphere: everything is cause and effect.' Consequently, 'becoming a vegetarian is the single most effective political action that anyone can take'.
He has answers to the usual objections. He knows that animals have the same emotions as humans, because he has seen ewes crying for their dead lambs; he knows that plants don't, because they don't have consciousness. Nor is he afraid to draw the most extreme analogies to make his point: meat- eaters are 'like people who claimed they didn't know the concentration camps were along the road, didn't know the Jews were being slaughtered'. He compares the pleasure that people get from meat-eating to that derived by James Bulger's murderers.
The use of such images can rile even those who might otherwise be prepared to concede that he has something of a point. But he probably has no need of tepid enthusiasm: his beliefs are absolute, and require absolute faith.
In the end, his insistence on vegetarian catering is hardly a great imposition. The crew will eat breakfast and dinner at home, and anyone who doesn't like the veggie lunches can go out. Cardiff offers plenty of alternatives. And he is in good company: when Paul McCartney or U2 or Michael Jackson go on tour, they all have vegetarian catering. It is, meanwhile, extremely likely that The Healer's caterers will produce something superior to the usual notoriously grim location food.
Newman's impact as a playwright evidently owes much to his passion. 'All our discussions are rooted in his great sincerity,' says his co-producer. 'You work with a lot of writers who write very good plays on the basis of research - but you know with Gordon that he lives it. Every script point we have an argument about comes from deep-seated conviction.'
Newman would never be able to make the kind of stipulation he has made on this film if he were not a highly respected, powerful playwright; he himself hopes that he is polemical without being didactic. And his plays are: he clearly finds their perspective from somewhere, though sometimes it is hard to see where.