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A yawn-inducing biopic of Jacqueline Kennedy, then? No. Jackie, the first English language film from the youngish Chilean director Pablo Lorrain (The Club, Neruda) is an astonishingly radically conceived portrait of Jackie focussed tightly on the immediate aftermath of the assassination of JFK, on November 21, 1963.

One week after his death, Jackie gave a famous interview, at the Kennedy Compound at Hyannis Port, to a journalist from Life, Theodore H. White (here played very cannily by Billy Crudup and called simply 'the journalist' in the credits), in which she first created the myth that Kennedy's presidency was like King Arthur's Camelot. Lorrain and his scriptwriter Noah Oppenheim do use this interview, which Jackie fiercely controls, as a framing device, but not to present a plain narrative of her life. Instead, we get close up, startlingly intimate, extended sequences, almost dreamlike in their intensity, not always in chronological order, of what she went through in such a short time after the shots were fired.

She is in the car speeding to the hospital trying to hold her husband's head together; in Airforce One, returning to Washington, while LBJ has himself immediately sworn in; breaking in to the room in which the autopsy is being performed; telling her two small children, the survivors of the four she had, that daddy has gone to heaven; taking charge of JFK's funeral and modelling it on that Lincoln, insisting on a march, not a motorcade; choosing where he should be buried in Arlington Cemetery; finally taking off that blood-soaked pink Chanel suit and finding more blood running out of her hair as she showers, her face contorted with grief; wandering completely alone through the rooms of the White House she had redecorated, as if in a trance, drinking, taking pills, trying on outfits; confessing her wish to die to an elderly Irish priest (John Hurt), their two faces together in extreme close-up, hers such an icon even in her grief, his so raddled; overseeing the immediate packing up of her family's belongings, as they leave their home; seeing a display of her own famous style in a shop window, as she passes by in the back seat of car.

Portman combines being completely dazed and numbed with extreme will-power and strong emotion, broken with grief but still at some level behind the eyes in command of herself

Natalie Portman is wondrously good in this part: surely her best and an Oscar contender. That she does such a remarkably good impression of how Jackie Kennedy looked and spoke is just the start of her performance. She combines being completely dazed and numbed with extreme will-power and strong emotion, broken with grief but still at some level behind the eyes in command of herself. She is both unreadable and riveting, sometimes seeming almost doll-like but always with an absolute grip on the new truths of her life.

"Nothing is ever mine, not to keep anyway," she tells the journalist. She knows exactly what he wants from her and she gives it to him, a full sensory account of what it was like to be with her husband as he was killed - and then she says sharply, "Don't think for one second I'm going to let you publish that." To the priest, she admits simply that she and her husband hardly ever spent a night together. "I lost Jack somewhere, what was real, what was performance," she says.

If Portman's performance makes the film, the secondary casting is admirable too, notably Peter Sarsgaard as Robert Kennedy, with some of his brother's potency but himself somehow already foredoomed, and Greta Gerwig, so warm as her personal assistant. But an even greater contribution is made by the choice of the highly original young British composer Mica Levi, who created the brilliant soundtrack for Under the Skin, to score the film: from the start, when discordant chords sound momentarily like a train charging past, it's an extraordinary piece of work, hugely enhancing the emotional impact of the whole movie.

Jackie doesn't resolve into any tidy interpretation of Jackie Kennedy or how she survived such days – yet it does give you insight into what the shock of sudden and violent bereavement, and with it total loss of position, might be like for any one of us. An absolutely unexpected achievement, this film, unlike anything seen before.

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