Inside the world of theatre's most reluctant hero

The greatest stage actor of his generation is practising an ultra-deep voice when we meet. That's quite endearing, as it's what wannabes tend to do when they fantasise about going on the stage. But Simon Russell Beale is practising it for his next starring role, as the millionaire arms dealer Andrew Undershaft in Shaw's Major Barbara at the National Theatre. He is, he says, modelling the voice, or at least its rich timbre, on the late Robert Maxwell.

At 47, with every rave review imaginable behind him, Russell Beale still has something of the wannabe's infatuation with the theatre. It surfaces when I mention his triumph in the National's Much Ado About Nothing, and the pool on stage into which he had to leap. "I lost confidence about the pool just before we opened. But when I did it, it was wonderful and warm. And to soliloquise from there was marvellous. You always do your best performances in the bath."

But the deep voice for Shaw emanates from a different concern. "I'm way out of my comfort zone. This will be the first alpha male I have played," he says. "You know I tend to walk on to a stage sideways." That's a lovely image, in the way (or maybe not in the way) of a man who has a first from Cambridge it gets to the heart of many of his roles.

Russell Beale has played a lot of failures, people disappointed with life and with themselves. He once posited the question of whether he always ends up playing failures or whether they become failures when he plays them. He can be touchingly self-deprecating. He's said of his private life that it used to hurt not having a partner, but he has "sort of given up on it. It's no longer a worry."

David Leveaux, who directed him in Jumpers, has said: "Simon has a self-image that is not exactly a love affair. And yet when he acts you have the sense that he is completely in his body: he is sensual and he has an ability to communicate thought and feeling on the same waveband. He has turned lack of self-esteem into an art form."

Next week, that changes. The part he will be playing is different from any in his career. "I usually divide my parts into two," he says, "the fuck-off parts and the I'm-sorry parts – Iago in the first camp, Hamlet in the second. But I've never really played an alpha male before. Perhaps Benedick in Much Ado comes nearest. You know, he is the best person I have ever played. He is such a good man. He does something extraordinary in going over to the women's side for moral reasons and risking his life. That was a big thing to do. And he says to Beatrice, 'I do love nothing in the world so well as you.' Can you imagine what it is like to have that said to you?" And for a moment he looks rhapsodic.

They are special, those Simon Russell Beale moments. There have been many in his 25-year career: the moment in the revival of Christopher Hampton's The Philanthropist at the Donmar when his character of an Oxbridge don has his hair ruffled by a would-be seductress, and he, in a chair facing the audience, adopts a look of abject horror; his Macbeth examining the dead body of Lady Macduff to try to understand the nature of death; and (my favourite moment) in Terry Hands's definitive production of The Seagull for the RSC when Russell Beale's rejected and desperate young Konstantin carefully folds his clothes before withdrawing to a room to blow his brains out. It's pleasing that he shares a conspiratorial look when I mention his Konstantin. "It's funny, I don't feel at all proprietorial about my roles. I went to see Ewan McGregor as Iago and I liked what he did. He was really honest. My Iago wasn't honest. You'd never have trusted me. But Konstantin is the one role I do feel proprietorial about."

He is known for bringing his own carefully thought-out approaches to a role into rehearsal, having as much say as the director, if not more. In Macbeth, it was his idea to be seated centre stage in the final act with the action coming to him, an almost helpless spectator in his own downfall. Was the heart-rendingly meticulous folding of the clothes in The Seagull his idea? "I remember standing in the rehearsal room thinking, 'I'm sinking into the blackness here.' I did ask my mum, a GP, and she told me that if there's a suspicion of suicide, first you smell for alcohol and then you ask if someone wears glasses as you don't kill yourself with glasses on...

"Actually, Chekhov decrees that there has to be two minutes' silence at that point and that's a long time to fill. I was desperately searching for things to do." Well, asking a great actor V C about their technique is always liable to result in a self-deprecating answer, or an angry one. Laurence Olivier snapped at Melvyn Bragg, when asked to reveal his technique, that Bragg wouldn't have asked a jeweller the secrets of his craft.

And some things certainly are a mystery, even to their creators. Stage presence is, for example. An indefinable magnetism onstage is something I have experienced with only two actors, Olivier and Russell Beale. But, now that Russell Beale seems to have universal acknowledgement that he is first among equals – his Much Ado performance even provoking a leader in his honour in a national paper – I do feel it is worth risking a brush-off to get him to examine and share his method.

Besides, with a person who has a perennial twinkle in his eye, displays bursts of laughter at a distinctly non-Maxwell pitch, and has a remarkably easy-going charm, any brush-off would always be polite, always reflect the courtesy he learnt at his public school – or big school, as he refers to it with the endearing lapse into phraseology of a time and class and upbringing that seems to belong to long ago.

As it happens, he's happy to share. To prepare for Shaw's arms dealer, he looks for "the things in the emotional life, see it as a play about a family whose father comes back after 25 years... Funnily enough, I was thinking about my own family and how it has changed. My great-grandmother ran a butcher's shop in Romford and her child did that rather English thing, educated at a private school and going to Cambridge. So I was thinking about that; granddad was a clerk in Romford and his grandchildren are doctors, consultants, opera singers and actors."

His rule for preparing a role, he says, is to "get rid of any preconceptions, and that's much, much harder than it seems. So, with Cassius you can't go into a rehearsal room thinking that this is a political manipulator. Cassius turned out to be incompetent, and the funny thing I found out; he threatens to commit suicide in every scene except the one he does commit suicide in, and I thought that's interesting, he believes in a republic so much he kills himself. In Richard III, it had never occurred to him, the grief he'd caused. He isn't a psychopath, like Iago. He asks to marry [and unite the houses of York and Lancaster] because he genuinely wants it to stop. You go to an emotional life you didn't expect to see.

"My technique is a puzzle. If I've got one thing, I've got a strong voice. It's not a beautiful voice but it's a strong one. I must have learnt to do that through singing and my years at the RSC. I think I don't have a good technique, to be honest. I've certainly learnt things I'm aware of using. Stillness and conservation of energy. I've a weakness for flashing energy around in a rather diffuse way... Stillness and concentration is the most pleasurable thing, even more than laughs. I enjoy the emotional landscape. I enjoy being aware of taking an audience through the journey."

Which leads to the big question; big as in Big. Perhaps it should be irrelevant, but Russell Beale has pretty much raised it himself. He said to my colleague Paul Taylor: "I hate my body. I hate it. I hate my looks. I hate my voice. So much of what I do on stage is really saying, 'Love me, despite the fact that I'm ugly.'" That was in an interview in which that same colleague unintentionally outed the actor as gay to his Independent-reading parents.

So, does his chubbiness rule out some roles? Can he never be the romantic hero? Will there always be the odd, decidedly odd, critic such as the one from the The New York Observer, who wrote of Russell Beale's Uncle Vanya: "Give me a thin Vanya. You cannot take a man seriously who looks as if he's pining for a sticky bun."

He sighs. "It has been happening for 25 years. I wish it would go away. I suppose it's mildly irritating if the comment has no relevance to the part, as with Vanya. Of course it upsets me sometimes. Someone once said I looked like a pig, and that hurt. I was never going to be Romeo. But Benedick gets the girl. I would love to do Coriolanus, but I don't think I could do that. I wish it wasn't an issue, but then it's an issue if you're very beautiful."

Actually, he's got me wrong; I raise the question not because I think it is an issue, but because I think it should not be. This man who has played a chubby Hamlet and a chubby Cassius, Shakespeare's "lean and hungry man", could play anything. He is surely wrong to rule himself out of playing Coriolanus. I tell him I would like to see his Antony (he demurs, but hints at a forthcoming Enobarbus). For me, his power and magnetism on stage means that he could play anything.

He stands, pulls up his shirt and pats his tummy, which is, as they say, too much information. "It's not small, but it's not as bad as it was." So what was his reaction when the director Deborah Warner asked him to play Cassius? "I remember it vividly. I said, 'You must be joking.' I was very large then. Two weeks into rehearsal I said, 'I don't want to be humiliated.' But she felt the emotional landscape in the man; this neurotic, passionate man was something she wanted me to explore."

Warner would have seen his power to isolate and convey the emotional heart of a character. Where it comes from, who knows? There has been speculation that it might have something to do with the death of a younger sister, Lucy, who was born without one of the chambers of her heart and died at the age of four. When he was 11, he was called out of a school lesson and given the news by his father. He once attributed his choice of PhD thesis – "The culture of death in Victorian Britain" – to lingering grief over Lucy's death.

Russell Beale did not grow up going to the theatre, a fact evidenced when I ask who his acting heroes are. No Olivier or Gielgud; he saw neither on stage. It is his contemporaries Alex Jennings (for his technique), Mark Rylance (for his imagination) and Tom Hollander he admires. His training was in singing and classical music. He is a hugely enthusiastic pianist, and speaks of being in Spamalot on Broadway with David Hyde Pierce (Niles in Frasier), also an obsessive pianist; they played backstage.

Russell Beale's life was never meant to take the direction of acting. His father was Surgeon General to the British Army, his mother a GP. He lived abroad when he was young – Singapore, Hong Kong, Germany – and, aged eight, became a chorister at St Paul's Cathedral School before going on to Clifton College in Bristol on a choral scholarship. "I acted at school a bit. As with most actors, it was a great English teacher at big school who was the spur. I did Desdemona at 14 for him. He was extraordinary. He was very puritan. I was going to be a doctor like my parents. He said, 'You can't do medicine, you've got to do English.' He got me to play Lear when I was 17. Doing the school plays and studying Shakespeare were the spurs. But before then, the headmaster at a previous school said to me, "Do the 'dogs of war' speech from Julius Caesar. I can remember where I was sitting, I remember the sun coming through the windows. I must have been about 10."

After school, he went on a music scholarship to Cambridge and to the Guildhall School of Music, but didn't finish that course. "I found London quite tricky to live in," he says slowly, "and I was just not happy. I needed to get out." That was pretty much arranged for him, as he was spotted at the Edinburgh Festival with a student production and turned professional, rapidly progressing to the RSC at Stratford and a quarter of a century of growing acclaim.

The paradox, of course, is that he is the greatest stage actor of his generation, yet 90 per cent of the population hasn't heard of him. That is theatre's problem rather than Russell Beale's, but the dearth of film and TV work means there will be little record of such a monumental talent. He seems immune to the allure of film. "I feel very deeply that the most exciting thing you can do is watch real live people on stage. I still get a thrill from going to the theatre. And that applies on the other side of the footlights for me. I just love being there." And that is our thrill, too.

Next up is Major Barbara. Russell Beale says that both he and the play's director, Nicholas Hytner, are "Shaw sceptic", and they have been cutting vigorously. "We felt it had to be cut down because it's so verbose. But tonally it is very elusive. It's funny but contains great serious thoughts."

Some in the audience may be Shaw sceptic, too. But for many in the audience, Shaw is not the reason we are going. We are going to see the best in the business perform.

'Major Barbara', at the National Theatre, London SE1 (020-7452 3000) from 26 February to 15 May, is part of the Travelex £10 season. 'The Independent' is media partner, and the National has reserved 150 tickets to the play for 'Independent' readers on Tuesday 22 April. Tickets, priced at £10, include an exclusive opportunity to meet members of the company after the performance. To book, call the above number and quote 'Independent Reader Offer'. Subject to availability