I first fell in love with her in River of No Return, in which she starred with Robert Mitchum and Rory Calhoun. The movie was released in 1954, and probably did not get to Ireland until the following year, so I was 10 when I saw it, at the Capitol Cinema in Wexford. The Capitol was a rather gaunt, barn-like picture palace, with a wooden floor and seats covered with balding plush that gave off a curiously doggy smell. But the glory of the place, at least in my memory of it, was the great scarlet curtain, fluted and fringed, that would open with a deeply suggestive swish as the house lights dimmed and the dark screen came to flickering life.

For many years that curtain was associated in my mind with Kay Weston, the saloon-bar singer Marilyn played in the movie – no doubt the shade of rich red and the sumptuous, silken folds seemed the very essence of sexiness, for a boy who as yet knew nothing about sex. The plot of River of No Return, loosely derived from Vittorio De Sica's 1948 film Bicycle Thieves, is a lot of hokum, of course. Mitchum plays a homesteader just out of prison after serving a sentence for shooting a man in the back, and Calhoun, an oleaginous smoothie born to play spineless villains, is a gambler who steals Mitchum's only horse. When a band of injuns attack, Mitchum, Monroe and Mitchum's young son are forced to escape downriver on a raft.

In the movie's saloon scenes Monroe wore some extraordinary belle epoch gowns that might, indeed, have been run up from surplus cinema-screen curtains, but throughout most of the film, as Mitchum manfully steers the raft through the rock-strewn flood and raging rapids, she is clad in skin-tight jeans and a simple white blouse with a plunging neckline.

Oh, that blouse. For a little boy not long weaned himself, breasts were the great desiderata, the high points of passion, the impossibly distant hills towards which all sensual yearnings tended. There is a scene in River of No Return in which Marilyn falls off the raft and is pulled out of the water with her blouse plastered to her skin. For weeks afterwards the image of this gorgeously dripping naiad was to haunt my nights, the sleepless hours of which I filled with an elaborate fantasy in which muscleman Mitchum and his pesky son were swept away in the flood, leaving it to me to rescue Marilyn and bear her to the river bank, where I would build a fire before which she might dry her jeans and her drenched blouse, the removing of which I had take place in lovingly slow slow-motion.

According to The Marilyn Encyclopedia, Adam Victor's splendidly exhaustive and marvellously illustrated compendium of Marilynia, the star hated doing River of No Return, and throughout the filming of it carried on a war of attrition with the director, the feral Otto Preminger. It was the usual story of late arrivals on set and deliberately fluffed lines; at one point, having hurt her foot, she insisted on having a plaster cast fitted, which held up filming for days while she hobbled about eliciting sympathy and posing for press pictures on her crutches. She was that kind of girl.

And who could blame her? The moguls who ran the movie world treated her foully. Gore Vidal used to remark in his sardonic fashion that Hollywood never destroyed anyone who was worth saving, but the cruelty meted out to Marilyn seems excessive even by the standards of the time and place. In a new biography, restrainedly entitled Marilyn: The Passion and the Paradox, Lois Banner, recounts how Shelley Winters watched some of the filming of River of No Return and described Preminger bullying Marilyn "into total immobility". At one point he told her that she should give up acting and go back to her "original profession". She described the movie as a Z western, and coming out of the cinema after seeing a preview of it she went round a corner to vomit. My 10-year-old self would have been sadly disconcerted had he known of these matters.

And yet, for all his fantasising, even that 10-year-old had an inkling of the reality behind the dumb-blonde persona, the creation of which was Marilyn's masterwork. What made MM so bewitching was the very fact that she was – that it was – all or almost all an act. There is the famous story of her walking with a friend down Fifth Avenue in a headscarf and raincoat, as ordinary as anyone else on the street, discussing fame and celebrity and what it was to be a star. "Do you want me to be her?" Marilyn said to the friend. "Watch." And taking off the headscarf and opening her coat to thrust out her chest, she went into the MM sashay, and within half a minute was surrounded by a baying mob of fans and autograph hunters. If the story is true, it gives the lie to her contention that it took five hours of preparation every time for her to "become" Marilyn.

The fact is, she was one of the 20th century's great clowns, whose clowning was intended not to make us laugh – though she was wonderfully funny – but to lose ourselves in fantasies of longing and desire. Other movie stars act the part of themselves, more or less convincingly; Marilyn created a wholly other version of herself, meant not to convince but to seduce. She was both Frankenstein and Frankenstein's monster, and it is our constant, subliminal awareness of this duality that makes her such a fascinating and compelling creature, even still, 50 years after her death.

Was she a good actor? Was she an actor at all, in any accepted sense? Her longing to be taken seriously as an artist, her ambition to play Chekhov, and Nora in Ibsen's A Doll's House, only add to the pathos of her story. She was probably as ill-served by those who assured her that she could be a "serious" actor as she was by the studio louts who told her she was nothing but a dumb blonde, and a slut besides. Lee Strasberg, the proponent and teacher of method acting, believed she had great talent. Banner writes:

"Strasberg realised that Marilyn's sweetness masked a deep rage, and he worked with her to bring it to the surface in order to deal with it. Her rage was so great that she used scatological language [Heavens! – JB] with him to express it … Marilyn soon came to idolise Strasberg as a guru and father figure. She offered herself sexually to him, but he refused her, realising that a sexual relationship might contaminate their work."

There is something at once sad and faintly risible here. By the time Marilyn threw herself and her hopes at the unprepossessing figure of Lee Strasberg, her image as MM the blonde bombshell was thoroughly reified, at least in the eyes of a mesmerised public. Having fashioned this more-than-lifesize image of herself, how could she expect to unmake it and start again, reappearing in the guise of a dedicated actor capable of taking on tragic roles? Could Grock have turned himself into Laurence Olivier?

How did she get away with the presentation of such raw sexuality? For no amount of airbrushing or euphemistic costuming could hide the fact that this was a real woman, with a shape – slightly dumpy, with plump thighs and a big bum – not all that much different from that of your mother, or your wife, or that girl down the street you always fancied. This was a large part of her attraction, for men, and possibly for women, too, that she was peculiarly ordinary, and yet a phantom out of the sweatiest of dreams. She began to make her career directly after the war, when America was busily getting its womenfolk out of the dungarees so many of them had worn in the armaments and aircraft factories and into the gingham aprons more appropriate to their true station in life. Marilyn was the one who was allowed to stay free, to remain in the wild.

If she could, she would have lived inside a camera. One suspects that for Marilyn, her image was more real than her self. The depth of her self-absorption was uncanny. A friend told of passing through her house and seeing her sitting in front of a mirror, gazing at her reflection, and then returning some time later to find her still there, still at gaze. "I'm looking at her," Marilyn explained. Few of us could bear such an acute awareness of our physical presence in our own lives; perhaps, in the end, she could not bear it, either. Yet she had a genius for cultivating the camera, and cameramen – and they were all men, with few exceptions; Eve Arnold, who took some wonderfully intimate, tender and witty pictures of her, was one of the few women photographers Marilyn trusted.

She knew from the start how vital to her success still photography would be – the photo magazines of the 1950s and 1960s were bought by the million. One of her first image-makers was Bruno Bernard, who one day in 1946 spotted the as yet unknown Norma Jean Dougherty on Sunset Boulevard, gave her his card – "Miss, this is strictly professional" – and told her he wanted to take some photo tests of her. She was still a brunette in those days – her dyed-blonde hair she would later describe as "pillow-case white" – but on the evidence of Bernard's pictures of her she already had a unique glow. Or is it just that it now seems unique? Her looks were not remarkable – no one with a nose like that could be described as a classic beauty – but she was entirely photogenic.

Just how passionately the camera loved her can be seen in Marilyn: Intimate Exposures, edited by Bruno Bernard's daughter Susan. He was present in New York on the day in 1954, during the shooting of The Seven Year Itch, when Marilyn did one of her most iconic scenes, standing over the subway grating and letting the updraft from a passing train – actually a wind machine positioned under the grating – to lift her skirts and show off her bare legs all the way up to her underpants. The pictures Bernard took of her are more revealing, and therefore less erotic, than the images that were allowed to appear on the screen; although Marilyn is at her most rawly sensual, what one feels most acutely when looking at her in these photos is a chill of melancholy.

Susan Bernard, in her fulsome style, describes Marilyn in this scene as "a patron saint, in the image of an angel bearing wings". Likewise, Banner opens her biography of Marilyn with a toot on a celestial trumpet: "In her white dress, white underpants, white high-heel slingbacks, and white earrings, Marilyn is a vision in white, suggesting innocence and purity. Yet she exudes sexuality and transcends it, poses for the male gaze and confronts it. Her billowing skirt resembles wings. She might be a guardian angel from the Christian tradition, and Aphrodite from the classical tradition, or a Nike proclaiming victory in poetry or war …"

Well, yes, sort of. But if Olympian comparisons are to be made, we might best say that the goddess Marilyn most resembles is Persephone, who in one of the most lovely of the Greek myths was carried off by Pluto to be queen of the Underworld, and though subsequently rescued by her mother was condemned to stay in Hell for half of each year, so that winters would end and the mortal world know the light of spring and summer. Poor Marilyn certainly shed a little radiance in a dark time. Only she had no mother to save her.

To order The Marilyn Encyclopedia for £15, Marilyn: The Passion and the Paradox for £15, or Marilyn: Intimate Exposures for £20 all with free UK p&p call Guardian book service on 0330 333 6846 or go to theguardian.com/bookshop