By Jocelyn Noveck, AP National Writer

It was only weeks before production was starting on "The Souvenir," the story goes, and British director Joanna Hogg and her friend, actress Tilda Swinton, still didn't have a lead actress.

The role of Julie, an affluent, twentysomething film student in 1980s London, was loosely based on Hogg herself. Swinton was slated to play the mother. Suddenly the two longtime friends realized they'd been ignoring the obvious choice: Swinton's then 21-year-old daughter, Honor Swinton Byrne, who'd appeared in one film as a child and had no plans to be an actress.

The result is a fairly smashing debut, a performance so genuine, detailed and lived-in that Swinton Byrne may have the film world begging her to reconsider her plans to study psychology and neuroscience at university.

One of Hogg's key assets here is having a real-life mother and daughter duo; rarely has a film portrayed so authentically the give-and-take, the push-and-pull that occurs specifically between a middle-aged parent and their young-adult offspring. "I think you should go to bed." ''I thought you might need this lamp I bought you." ''Mum, I need some money." ''Oh. Again?" It all feels so natural, it's as if we're peering through the window blinds with binoculars and should be arrested for spying.

Another reason the film has such an authentic feel is that much of the dialogue is clearly improvised. The purpose isn't always clear; you need to sit back and slow down your pulse to watch this type of filmmaking. Hogg has a plan, but it unfolds slowly.

As we meet Julie, she's pitching her first feature, based in the shipbuilding city of Sunderland in northern England — a world far from her own privileged corner of Knightsbridge in London.

Julie is all earnest ambition, but lacks assurance. Then she meets Anthony, her polar opposite: all self-satisfaction and intellectual swagger, he takes swigs of his cigarette and makes pronouncements on life, art, and Julie. The young man works for the Foreign Office — or so he says — and, as portrayed by roguishly handsome Tom Burke, is both comforting and frightening at once.

It begins platonically, with discussions about art. When Anthony comes to stay in Julie's London flat, the two share her bed — chastely — and engage in charming friend-banter about who's taking up more room.

But it's also clear, at least to us, that Anthony's eventually planning to seduce Julie, and we wince in anticipation. We especially cringe when he brings her some sexy lingerie as a gift, launching the sexual relationship we know will bring her pain. And that's even before we learn that Anthony's a heroin addict.

We can only steel ourselves as we watch Julie fall ever deeper, against her own judgment. When she comes home one day and he informs her they've been robbed, with all her jewelry gone, it's somehow SHE who ends up comforting HIM. At another point, when he admits to some reprehensible behavior, she actually apologizes for being angry.

Meanwhile, Julie's borrowing money from her well-meaning mother, so she can give Anthony money. Anthony falls deeper into the scourge of his addiction, and Julie can't find a way to shake off his hold. It's excruciating to watch.

Yet another reason for the genuine feel of Hogg's work stems from her acute attention to physical detail. She's based Julie's apartment on her own student digs, recreated down to the views from the windows. The soundtrack is full of period-appropriate gems like Joe Jackson's "Is She Really Going Out With Him?" — definitely an appropriate question here.

"We can all be sincere, but what's it all for?" Anthony says to Julie at one point, referring in a typically snobbish way to her film aesthetic.

But sincerity is what anchors this film — especially Swinton Byrne's astonishingly sincere performance. Bring on the sequel. And maybe neuroscience can wait.

"The Souvenir," an A24 release, is rated R by the Motion Picture Association of America "for some sexuality, graphic nudity, drug material and language." Running time: 119 minutes. Three stars out of four.

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