During the filming of F Scott Fitzgerald's novel The Last Tycoon in the mid-1970s, the director Elia Kazan told Robert De Niro he wanted him to give the movie's eponymous hero a pensive, distracted air. Learn your lines, counselled Kazan, but think about something else while you're saying them. "Shit," said De Niro's co-star Robert Mitchum, "I've been doing that for years." Given that Mitchum was famous for knowing a whole script after one read-through, he may not have been joking. With a memory that good, he could afford to let his mind wander. No actor ever looked as relaxed in front of the camera. As Philip Marlowe in Michael Winner's remake of The Big Sleep (1978), Mitchum has to tell a client that his daughter "tried to sit in my lap - while I was standing up". It was a great gag when Humphrey Bogart cracked it in Howard Hawks's 1946 version; but it punches above its weight the second time around, because we suspect that whatever Mitchum did, he did it lying down.
That said, Mitchum worked extraordinarily hard. Accounts vary as to how many pictures he made, but there is no doubt that they number more than 130. Even granting Mitchum the honour of a two-month retrospective, as the National Film Theatre in London is doing, is merely to scratch the surface of his output.
Mitchum, who died in 1997, would have been baffled by the attention. When Martin Scorsese directed him in the remake of Cape Fear (1991), he told his star he had seen every film he'd ever made. "Beats me," said Mitchum. "I've seen about seven of 'em." Unlike many a lesser actor, Mitchum never coveted a reputation. John Huston, who directed him in Heaven Knows, Mr Allison (1957), thought he would have made a great King Lear. Charles Laughton, who directed him in The Night of the Hunter (1955), thought he'd have been a wonderful Macbeth. Broadway wanted him to replace Marlon Brando in A Streetcar Named Desire.
But Mitchum wasn't interested in such highfalutin talk. He chose his roles on the basis of how much time they would allow him to go fishing. "If you can phone it in," he joshed Michael Parkinson in a 1972 interview being screened by the National Film Theatre, "fine." Yet Mitchum never once phoned in a performance. Even in his worst films - Joseph Losey's Secret Ceremony (1968), say, or David Lean's Ryan's Daughter (1970) - no blame attached to him. He might not always have done it in the best pictures, but he could always be relied on to give his best.
What his best was, however, is a tough question. Mitchum was one of the first actors for whom less was more, and it can be hard to get a grip on precisely what he did in front of the camera. One of the reasons he was film's best Philip Marlowe (in Dick Richards's Farewell, My Lovely, 1975) is that he threw away every line. Dick Powell and Robert Montgomery had tried to trowel significance into Marlowe's side-of-the-mouth sniping, but Mitchum's indifference to intonation let Chandler's wit do the work on its own.
As with dialogue, so with action. To watch Mitchum up against Marilyn Monroe in Otto Preminger's River of No Return (1954) is to see a contest of both talent and style. Monroe is all panic and flutter - a woman in motion even when she ought not to be. Mitchum, by contrast, is the still centre, the man who abjures action because he knows Preminger's metaphoric river is carrying everything along. Playing the heavy in the original Cape Fear (1962), Mitchum has little to say, but his loose-hung jaw and dead eyes tell us far more than all of Gregory Peck's lawyerly lectures.
Perhaps the best actors do a lot less than we tend to give them credit for. Up close, a great star can look more like a black hole. Orson Welles once watched Gary Cooper filming and thought him a nonentity: "He almost didn't seem to be there . . . You'd see him working on the set and you'd think, 'My God, they're going to have to retake that one!' . . . And then you'd see the rushes, and he'd fill the screen." If Cooper "almost didn't seem to be there", then Mitchum is the invisible man. "I've been accused of 'coasting'," he once said, "but there are some parts you cannot do anything else. There is literally nothing to do but be there."
Are we talking about acting, then, or rather a heightened sense of being? Is the biggest trick great actors pull to make us believe that, unlike the rest of us, they are content within their own skin? There is evidence that what passes for acting is no more than the projection of our own fears and fantasies. As far back as the 1920s, before the invention of sound, a Russian director and theoretician called Lev Kuleshov argued that the idea of an aesthetic of film acting was hooey. Editing, he argued, was what counted.
To prove the point, he constructed a short film that consisted of a series of images of objects - a child at play, a woman's corpse, a bowl of soup - intercut with another series of images of an actor's face reacting to the preceding object. Having shown the film to an audience, Kuleshov asked them to describe the performance of the actor. The actor, they said, looked happy watching the child, sad looking at the dead woman and hungry looking at the soup. At which point Kuleshov told them that the shots of the actor were all the same - they had read the successive emotions into his face because of whatever image they had seen preceding it. Kuleshov concluded that meaning was never inherent in a single image, but was constructed by the movement from one image to the next.
Mitchum probably never read Kuleshov's theories, but he was none the less one of the first to have an instinctive understanding of the Russian's aesthetic, grasping that a performer can say as much by breathing in as by breathing out.
The NFT programme argues that Mitchum was "the bridge between Wayne and Brando". Indisputable as that is, it also marginalises his importance. Mitchum is the hinge between the sharp-eyed, whirling-armed histrionics that even the subtlest of Hollywood stars inherited from the stage and the stately, stylised naturalism that has been a movie standard since the early 1950s. There is acting pre-Mitchum and acting post-Mitchum, and they are not the same.
Mitchum also understood that no actor, no matter how good, could do everything. Like singers, actors have a given range. Training can expand it, but not much. Moreover, the industry has no interest in stretching its players. Films are written (or rewritten) with a specific actor in mind. Preacher, marine, painter, cow-poke, private eye, troubadour, gunslinger: Mitchum played a more varied roster of roles than you might remember, but we all know what is meant by a "Robert Mitchum picture". This is not a criticism. In our age of auteurist studies and directors' cuts, we forget it is actors and actresses who draw us to the movies. Acting was once waggishly defined as pretending to be somebody else, but the recognisability of a star turn such as Mitchum gives the lie to that notion. Acting is in fact the art of pretending to be yourself. "I have only two styles," Mitchum once quipped. "With horse or without."
He was always disparaging about what he saw as his limitations, but because acting came so easy, he couldn't believe it was a gift. "Baby," he tells the gangster's moll, worried about their plans to run away together in Out of the Past (1947), "I don't care." His best biographer, Lee Server, used the phrase for his book's title, and the image of Mitchum as stoic stumblebum persists. But doper and drinker though he undoubtedly was - filming Vincente Minnelli's Home From the Hill (1960), George Peppard, who had been at the Actors Studio, asked Mitchum if he had studied the Stanislavsky method; Mitchum said he preferred the Smirnoff method - Mitchum did care. He just wanted to seem not to. Like Gene Kelly, he thought that if it looked like you were working, you weren't working hard enough.
The Robert Mitchum season is at the National Film Theatre, London SE1 (020 7928 3232) until 31 August
Christopher Bray's critical biography of Michael Caine is published this autumn by Faber & Faber
Four stars influenced by Mitchum
Marlon Brando Brando's mould-breaking performance in A Streetcar Named Desire owed plenty to the legions of impregnable, implosive brutes Mitchum had been serving up on the silver screen for the previous half-decade.
Michael Caine Caine's dour introspection owes far more to Mitchum than, say, Cary Grant. Harry Palmer's truculent self-regard was a Sixties update on Mitchum's existential defiance, while Caine's ability to render any line gnomic and unreadable could not have come about without Mitchum's meaning-heavy monotone.
Clint Eastwood Monumental, marmoreal, taciturn, immobile - Eastwood has carved a career from retooling Mitchum's frosty withdrawal into the stuff of heroism. Though he has never dared venture into the darker waters Mitchum so gracefully swam in, he has held faith with the belief that the actor's job is to be, rather than do.
Robert De Niro In films such as Cape Fear and The Night of the Hunter, Mitchum proved that it was possible for a leading man to play evil characters and not lose his audience. Until recently, De Niro played nothing but monsters of machismo - including the Mitchum role in Scorsese's remake of Cape Fear.