The drop-your-popcorn moment in Ian Holm's new film occurs when the Emperor Napoleon stumbles, dethroned and demoralised, into the grounds of a lunatic asylum on the outskirts of Paris. This modest green space is dotted with fruit trees and rose bushes - and between them, a dozen men in cockaded hats, each groping inside his jacket as though he were wishing he'd not had the crevettes. Although Holm's Napoleon is the genuine article - escaped from St Helena, living incognito in France and nursing an ambition to regain the Emperorship - he will never be able to resume his old identity. The world believes that he is dead. The moment he announces himself, he'll be just another five-foot-nothing madman with delusions of grandeur.
The Emperor's New Clothes offers Holm a rare starring role in a medium that has customarily been content to use him for the purposes of support: sometimes as a kite-mark of quality, sometimes as a fig-leaf for a moribund production. It's the price, I suppose, of versatility and a vicious dislike of sitting around doing nothing. "The older I get," he tells me, when we meet - on his birthday - in the library of a well-upholstered London hotel, "the more I realise that work is the most important thing. Even more important than love."
Holm has been married three times - his decade-long match with the actress Penelope Wilton was dissolved in 2001 - and has three children by two of these former wives and two by the photographer Bee Gilbert, with whom he lived between 1965 and 1976. But the fifty-odd films on his CV make the point for him. He was chilling as the android undercover agent in Alien (1979) - nobody forgets the scene in which his decapitated head issues its last words through a stream of milky effluvium. He brought moving delicacy to the role of the cuckold displaced by Rupert Everett's pretty-boy toff in Dance with a Stranger (1985). He buzzed with quiet power as the pioneering mental specialist in The Madness of King George (1994), though the plot set him up for a fall. He was triumphantly ludicrous as Napoleon (again) in Time Bandits (1981) and Polonius in Zeffirelli's Hamlet (1990), more convincing than he had any right to be as Andy Garcia's dad in Night Falls on Manhattan (1997), and impressively Middle Earth-weary in two out of three of The Lord of the Rings trilogy.
His ambulance-chasing lawyer in Atom Egoyan's The Sweet Hereafter (1997) is, however, the real jewel of his career: a shabby legal evangelist desperate to persuade the parents of children killed in a bus crash to sue for compensation. If you've seen the film, you'll almost certainly remember the scene in which he pleads his case to a bereaved couple by scuttering across their living-room floorboards in an attitude of supplication.
Holm's story begins within the walls of a nineteenth-century lunatic asylum in Ilford, Essex. He was born in 1931, the son of Dr James Harvey Cuthbert, superintendent of the West Ham Corporation Mental Hospital and a pioneer of electric shock therapy. His mother, Jean Holm, was 43 when she became unexpectedly pregnant with a second son. "I was just an afterthought," he reflects. "They were funny old people. Removed and elderly. I never really knew them." He suspects that his mother - whose family was connected with the White Horse whisky company - believed that she had married beneath her, and he knows only the vaguest details about his father's background - though he did have an Uncle Sidney, "who went out to British Honduras and was blown away by a hurricane."
Holm had no more intimate a relationship with his elder brother, Eric, who died of cancer in 1944 while serving with the Scots Guards in Egypt. "He was 22, which is a ridiculous age to die of cancer. Before it became fashionable. My parents never got over his death."
Despite the Gothic trappings, this wasn't, he insists, a miserable childhood. "Most sane people would have got out very early," he concedes, "but I was actually very happy in my little cocoon." That the family home adjoined the hospital seems to have had little impact upon Holm's domestic life. "The only patient I really remember was Mr Anderson, utterly harmless, but fascinating nevertheless. He used to wheel this barrow of soil from point A to point B, and then spend the rest of the day picking up each grain and putting it back again."
Dr Cuthbert's work as a member of the board of Broadmoor, however, made a much more forceful impression on his young son. Holm recalls his father's meetings with Ronald True, a well-to-do killer of the early 1920s who murdered a prostitute named Olive Young by stuffing a towel down her throat and strangling her with a dressing gown cord.
"True was an incredibly charming, erudite sort of gentleman," Holm recalls. "When my father appeared at the door of his cell, he would say: 'Would you care for a game of chess?'" Another name nudges his memory. "Around the same time there was also a young boy called Jacoby who must have been completely insane. He clubbed a woman over the head with a hammer." (Henry Julius Jacoby, I later discover, was a pantry boy at the Spencer Hotel who supplemented his income by robbing the guests - one of whom had the misfortune to wake up as he was rifling through her valuables.) "Jacoby was hanged, but True wasn't," says Holm, "and Jacoby became a kind of cause célèbre: why should one hang and the other not?" Unlike True, Jacoby was unable to use insanity to save him from execution. Perhaps their cases taught him something about the nature of insanity - or acting.
A place at Rada provided Ian Holm's passport from the asylum. Other institutions claimed him once his three years of study were over: a year of national service, and 13 at the Royal Shakespeare Company under the tutelage of John Barton and Peter Hall. His RSC years concluded with a trio of dramatic events - divorce from his first wife, the inauguration of his ten-year relationship with Bee Gilbert, and a triumphant run on Broadway in Harold Pinter's The Homecoming. It was also in this period that he secured his first significant film roles. "John Frankenheimer," he notes, "was one of the first directors who decided to actually come to Stratford and see a play." For The Fixer (1968), Frankenheimer sent Holm to Budapest to appear opposite Dirk Bogarde and Alan Bates.
For Holm, it was the meeting with Bogarde that was important. "We became pretty good friends for a while. He was deeply in love with my inamorata of the time." For several years, Bogarde and his partner Tony Forwood made favourites of Holm and Gilbert. "They tended to do that sort of thing. We joined the entourage for a year or two. Dirk was extremely bitter about England. I could see why he felt that not enough credit was given to him for proper acting, but he could often be very unkind. Very acid." You suspect a few drops of Bogarde's vitriol may have been hurled his way.
Holm is not a reticent interviewee. During our time together, he makes only one comment that he asks me not to commit to print. He tells me that he dreams about having the ability to fly, that he lost his virginity at the age of 24 and that he recently went to see his Lord of the Rings co-star Ian McKellen in Strindberg's Dance of Death, and was quietly appalled. ("I thought he was dreadful - far too tricksy, rather weak.") He tells me about his friendship with a rich American woman who loans him her apartment in Venice from time to time, supplying him with a guide in the form of her spookily precocious nine-year-old son. ("He travels everywhere by taxi and even knows which hotels serve the best prawn cocktail," Holm muses. "I hope that he doesn't end up a heroin addict.")
At the same time, he is no green-room raconteur. His speaking voice is as soft as a flurry of snowflakes; sometimes I have to lean forward to catch his words. He drifts towards the ends of sentences and smacks his lips to mark their progress. "What shall we talk about now?" he asks, gently. "Stagefright?" I suggest.
Although the theatre has, historically, been more willing than film to bill Ian Holm above the title, an anxiety attack during a performance of The Iceman Cometh in 1976 scared him from the stage for the space of 17 years. Down came the curtain, on went the understudy, and Holm did not venture back on the boards until the 1993 revival of Harold Pinter's The Homecoming. "Low self-esteem," he notes, "rides high in my system."
The Iceman incident produced a diagnosis. "They told me that I had a chemical imbalance that was only curable with drugs," he explains. "And because of my father and all that, I thought, okay, I'll take them." He was prescribed the anti-depressant Triptizol, and devoted his energies to film work. In the last decade, he's dared to return to the theatre - tackling King Lear and Pinter - but still suffers the occasional relapse. He pulled out of the National Theatre's production of Shelagh Stephenson's play Mappa Mundi at an advanced stage of rehearsal. "I suddenly clammed up," he confesses, "and thought: 'I can't do this,' and walked out. At least it didn't happen on stage. It happened to me during The Homecoming as well. I felt I couldn't even walk down the street."
What triggers these moments of insecurity? "It used to be linked to my domestic life," he muses. "I always remember Bee saying to me: 'Don't blame your bloody nervous breakdown on me!' Sometimes I just feel incredibly vulnerable, for no particular reason. But fortunately it has never happened in front of a camera. That's always been my great dread, because I can't really do anything else. I don't really have any other strings to my bow." How does he feel during a close-up, when the camera might move as near to his face as an optician using an ophthalmoscope? "I love close-ups. Odd, isn't it? I've never thought about that before. I love the fact that during a take, everyone - the sparks, the chippies, everyone on set - is silent and concentrating on that moment. As you rightly say, if anything it ought to be more alarming, thinking of all those millions of people who will see the film. But it doesn't have that effect on me."
Let's hope he keeps his nerve, and remains in the gaze of the movie camera, under observation - and nothing like Napoleon.
'The Emperor's New Clothes' is released on 30 January