Weight problem: Robert De Niro will be 50 on 17 August. He is one of the great screen actors of the age, established forever by Mean Streets, Taxi Driver and The Deer Hunter. So why hasn't he done anything that good for a decade or more?

FIFTY is an age at which boxers and baseball players are gone, barrels of blurred anecdote hanging round the gym. Yet it is also the age at which politicians and businessmen can reckon on the next decade bringing the greatest challenge. Actors must somehow carry so many ways of being 50 inside their failing skins. And actors may observe as they try to stay in work that at 50 Gary Cooper did High Noon, Olivier was about to be Archie Rice on stage, Dirk Bogarde played Death in Venice and Cary Grant was up for To Catch a Thief.

I don't see Robert De Niro in a cowboy hat, braving it out as midday approaches, pledged to defend a worthless town. Nor is he quite the man to radiate futile desire in a summer of plague as Mahler brings tears to his made-up eyes. And if Grace Kelly had flirtatiously offered De Niro a leg or a breast from the picnic basket, To Catch a Thief could have taken a very unexpected turn (just as, a few years later, Hitchcock let Psycho jump the rails and head into the heart of modern horror). However, shift Archie Rice from music hall to vaudeville and De Niro is just the stoic, poker-faced comic who could do The Entertainer anew - who else is so good at giving us a joke as a prelude to some devastating attack? I can see that Archie Rice at bay on a shabby stage, ready for murder or, like Samson, strong enough to pull the house down. There's a glimpse of that performer in the opening and close of Raging Bull.

Is this any way to pay tribute to an actor, to start wondering about parts he's never played, and never will? Well, yes, it can be, for this reason: we see actors after their decisions, looking back. The resume of a career has a line, a sense of progress. Mistakes may have been made. But the certainty of work done is inescapable. Real actors never know that state of mind. They are always considering possibilities, hoping for call-backs, and wondering if they can fit in x before y is ready - but don't bank on y ever happening (because its money seems soft), so don't let a, b, c or d think you're not interested (and, anyway, on c - which we know is garbage - they are talking dollars 8m).

We seldom know a fraction of what actors nearly did. De Niro studied hard to play the role in The Gambler (1975) eventually given to James Caan. He pulled out of The Unbearable Lightness of Being (1988) late in the day, letting in Daniel Day-Lewis. For years Harvey Keitel was sustained by the grievance that most of what he was offered was stuff Bobby had turned down. There's talk now of a Godfather IV which would go back to those years in the Vito Corleone story when De Niro became Marlon Brando. If you were hearing the story of Scent of a Woman for the first time - blind military martinet at the end of his tether - would you say Pacino, or De Niro? Will Bobby ever do Gershwin? He looks like him. Whatever happened to that exotic South American picture De Niro was going to do with Quincy Jones and any one of a dozen leading black actresses and singers - was that c? And now, De Niro as the monster in Frankenstein?

The great performances come out of chaos, and most movie stars are demons of mixed hopes, insecurity and indecision. Al Pacino won every award going for Scent of a Woman, and he came at last into that strange but definite kingdom, in which a critics' actor is loved by the public. Yet the obsession in his life is a short movie which he paid for himself, which he carries around with him and will only show to small groups, personally - a thoroughly implausible, yet deeply revealing picture called The Local Stigmatic, from a play by Heathcote Williams. Actors are crazy; they have to be to do what they do.

De Niro's justifications for feeling on the edge are not hard to find. He is one of the great screen actors of the age - he would be established forever because of Mean Streets, The Godfather Part II, Taxi Driver, New York, New York, The Deer Hunter, Raging Bull and . . . well, OK, let's stop there for a moment. There are two Oscars (Godfather II and Raging Bull) and two other nominations (Taxi Driver and Deer Hunter) in that list, as well as the unquestioned admiration of his peers and his public. But he has never really had a smash hit at the box office that depended on him. This is tricky to prove - The Deer Hunter and Godfather II were enormous successes, but they were built on the story, a group of actors, the daring of the concept. Whenever De Niro has made a picture that had to fly on him alone - Taxi Driver and Raging Bull - he has had every review his dreams could write, and a modest audience.

Why? Largely because Travis Bickle and the Jake La Motta of Raging Bull are not everybody's fantasies. There are actors who, for 20 years, in good and bad pictures, have enjoyed the deep, daft fondness of the crowd - Nicholson and Redford are the most notable examples. They are fine actors; they have made some excellent pictures. But they have got hold of public love because of some helpless transparency or generosity in them. They like to be loved; they believe they deserve it. Nicholson can play a heavy - Torrance in The Shining or the marine officer in A Few Good Men - he can threaten other characters, chew scenery and do his full mad act, but nothing shakes the public free. They just love Jack doing his thing.

De Niro is not like that. He does not do television public-service announcements to save the condor, the trees and the air (like Redford). He does not happily display himself for TV coverage of basketball games (like Jack). He hardly ever does interviews. He does not make himself available as a public figure. And he never instills more unease in us than when he smiles. That's when you see the knife. After more than two decades' hard work on screen, in a re-make, De Niro came on as Max Cady in Cape Fear and was so all-round nasty and disturbing that one of the chief questions viewers took away was: 'Why did De Niro do that film - is he really like that?'

De Niro does not try to explain. People close to him say that would be beyond him. He is uncommonly inarticulate and private, which likely means that his work has a danger that threatens him more than it does us. By contrast, Nicholson can play a holy terror while conveying the starry sub-text, 'Don't worry, Jack knows this is wild. This is acting, kids]' There's a moment in Raging Bull when the needs of a role help us understand an actor. La Motta is being thrashed and bloodied by Sugar Ray Robinson - but he won't go down. De Niro is like that. He can't take a dive to get into our sympathies; he will not yield in his own implacable impregnability. There is a look on his face - in the smile, especially - that says, you will not know me. I will never go down for you. This is rare denial in an actor, and could yet prove self-destructive.

It was difficult for years to think of any movie in which De Niro played for sympathy. Bang the Drum Slowly (1973) was the one instance, though even there his dying baseball player had appeal far more because of the situation than because De Niro reached out for pathos. What made Taxi Driver so remarkable was De Niro's refusal to make Travis simply a monster or a lost soul. By the end of that persistently ambiguous film we did not quite know whether he was insane, a saint who had gone astray, or a mocking commentary on American celebrity.

Taxi Driver is most dangerous in its coda. After the slaughter, we somehow suppose that Travis is dying or that he will be confined for the rest of his life. But he comes back; he is still driving a taxi. He is free and every bit as confined in loneliness as he ever was. Is the ending real, or a dream? However one answers, De Niro has become the movie - the face in his own rear-view mirror. No other actor could be so powerful and yet so recessive. The American energies of idealism and violence fuse, and the influence has been felt not just on screen, but on the streets. 'You talking to me? You talking to me? You must be, 'cause I'm the only one here.'

In the second half of the Seventies, De Niro had an amazing run. It was not just Taxi Driver (1976), but the uncanny skill with which he studied Marlon Brando in the original Godfather and then projected that man's younger self. There is a command of gesture that must have startled Brando - and shows how strong an element of mimicry (or body-snatching) there is in De Niro's spontaneity. There was The Deer Hunter (1978), so loose a project it relied on De Niro's sergeant-like authority to hold things together. There was Raging Bull (1980), in which the actor made himself both leaner and harder than any movie athlete and a grotesque, bloated wreck. De Niro put on the weight, and gradually took it off, in ways that showed inhuman discipline and lack of self- regard. Few actors would have been so driven.

There were failures in these years, too. De Niro went to Italy to make 1900 (1976) for Bertolucci; he tried to be the frail Jewish producer Monroe Stahr in The Last Tycoon (1976) - and lost a lot of weight. And with Scorsese he did New York, New York (1977), striving to seem like a real saxophone player. The film was a box-office flop, but I suggest that it is one of De Niro's greatest performances - as Jimmy Doyle, the musician who marries the singer (Liza Minnelli), who has genius as player and seducer, but who loses interest once he has conquered. His Jimmy is ordinary next to Bickle or La Motta: he is a circumspect, wary paranoid who can pass in the world. Yet he is crazy in his own armoured solitude. He cannot be reached, broken or touched. He cannot really be in love. New York, New York was made in a time of personal frenzy for Scorsese and Minnelli. It may settle in eventually as one of the director's most distressing works in its admission of how far the hero - artist, hustler or boxer - goes his own lonely way.

There has never been a bond like that of Scorsese and De Niro. For years, Scorsese seemed unable to trust his vision to any other actor. He is as talkative as De Niro is reserved. And if De Niro is truly hard within, Scorsese, I think, is a smart, sensitive kid who likes to imagine himself keeping company with goodfellas. Their relationship would make a great movie for both of them. One other point: when they worked together it was easier not to notice that neither of them was very comfortable with, or even interested in, movies with women.

De Niro has tried love stories, and they have not taken. In Falling in Love (1985), the chemistry between him and Meryl Streep is less compelling as the centre of the film than it had been as a sub-plot, barely even in the script, in The Deer Hunter. He was no better with Jane Fonda in Stanley and Iris (1990), or Uma Thurman in this year's Mad Dog and Glory. Actors who won't go down may not be made for love stories. But careers are very vulnerable without that mainstream material. New York, New York showed us a love bound to fail, and there was an undertone of sexual fear in the picture, a wandering that preferred pick-ups to relationships. Since Raging Bull, it can be argued, both De Niro and Scorsese have faltered. Don't misunderstand. The King of Comedy (1983) is a film that grows more intriguing with time, yet I can't lose the thought that actor and director were experimenting with it. GoodFellas (1990) feels like stale territory. De Niro is very good in it, but overshadowed by the explosive Joe Pesci. Then came Cape Fear (1991), a picture that smacked of Scorsese's calculated attempt to have a big hit at last, but which left a foul taste in the mouth.

Cape Fear is a re-make that needed more alteration. Its violence is the nastier for being so predictable and gloating. The best scene in the film is the long, seductive talk between Cady and the teenage girl (Juliette Lewis), his prime target. It brings out a gentler, needier De Niro - he does understand this girl - who might have been more interesting than the devil Cady. Suppose that the real threat Cady carried for Nick Nolte's lawyer was to win the heart and mind of his daughter. That might have been richer and more disconcerting. As it is, Cady only illustrated De Niro's readiness to be Satanic (a mood first touched on in the foolish Angel Heart, 1987).

Today, De Niro has his own production company, TriBeCa, and is co-owner of a good restaurant in that part of lower New York City. He is in demand, even if the business long ago decided he was not a crowd-pleaser. He will do the monster for Kenneth Branagh in Frankenstein, and I hope Branagh can keep up with De Niro's intense, unspoken commitment. Mary Shelley's creature was never Boris Karloff's. There is room here for a stunning performance. What man is lonelier than the one just made from missing parts? What role says more about the desperate nature of acting?

It remains to be seen how well De Niro can handle his next decade, granted that he does not swim in the mainstream. In the past 10 years or so, too much of his work has been minor: the priest and Robert Duvall's brother in the moody True Confessions (1981); as lost as everyone else in The Mission (1986); a big, bad, surface-deep Al Capone in The Untouchables (1987); very funny with Charles Grodin in Midnight Run (1988); in Jackknife and We're No Angels (both 1989) for no clear reason; a victim of McCarthyism in the decent but dull Guilty by Suspicion (1991); simple supporting parts in Brazil (1985) and Backdraft (1991); the step- father in This Boy's Life (1992); nowhere near as good as Richard Widmark in the forlorn remake of Night and the City (1992); doing Mistress (1992) for a friend; playing dumb for Bill Murray in Mad Dog and Glory (1993). It isn't that all of those were poor pictures, but they hardly needed or exercised De Niro's size and potential. Of his recent films only Awakenings - a dormant life briefly restored to opportunity - gave him the awesome task he needs (and the Oscar nomination that used to be virtually automatic). Without it, he shows the boredom of genius stranded in a cautious business.

It is telling how Harvey Keitel's career has flourished lately. Once upon a time, Keitel and De Niro were young equals, and in Mean Streets it was Keitel who had the lead role. Then De Niro raced ahead. But now, with unnerving performances in Bad Lieutenant and Reservoir Dogs, dead-on professionalism in Mortal Thoughts, Bugsy and Thelma and Louise, and the adventure of The Piano, Keitel has become as ubiquitous and as potent as De Niro 15 years ago. Time for them to be teamed again? That might be the best chance of De Niro recovering his danger. Suppose, for instance, that his monster in Frankenstein has the amazement of Awakenings, the sombre menace of Max Cady, the sexual paranoia of Raging Bull . . . and suppose he lives in a taxi.

David Thomson's 'Showman: The Life of David O Selznick' is out now (Andre Deutsch, pounds 20). He is preparing a revised edition of the classic 'Biographical Dictionary of the Cinema'.

(Photograph omitted)