At the start of Life of M, the new novel by the British-Canadian writer Rachel Cusk, a writer asks a friend who is a famous actor if they can write their autobiography. The actor – the “M” of the title – agrees and then says: “Would you just make it up?” And so, rather than write a conventional account of M’s life and career, the author delivers a series of scenes seemingly aimed at revealing what it is to exist in the glare of celebrity and in a world cushioned by wealth.

M, who has been famous since childhood, is depicted as detached and artificial, a woman whose existence is filtered through the lens of how others see her. From a young age, she understands that “reality can be engineered”. Thus, her efforts to do normal things like take her children to school, walk her dogs or visit a cafe are just that: mechanical efforts at normality.

This 13th novel by Cusk arrives in a squall of conjecture and gossip surrounding the identity of M, who has been assumed by some to be Natalie Portman, with whom Cusk is acquainted (she has appeared on , the Instagram account where the actor flags her favourite books). It also raises questions about the ethics of this less-than-flattering character study, which can be filed under autofiction, a genre that murkily blends memoir and fiction.

Cusk – one of the most acclaimed writers of the past 20 years – has form here. An early print run of her 2009 memoir was destroyed after she was threatened with legal action by a man who recognised himself in the book and cried foul. In 2017, the author Damian Barr, who met Cusk at a literary festival a few years prior, observed a version of himself in her book . Barr archly noted that Cusk’s veil of fiction was “thinner than a thin thing. Diaphanous. Gossamer.”

It’s understandable why Life of M has been considered by the press to be based on Portman given the bones of the story. M rose to fame as a child after starring in a movie with an older male star, as Portman did in 1994’s . As an adult, M played the lead in a movie about a ballet dancer who has a breakdown, as Portman did in 2010’s . M has appeared in a perfume ad that sees her “wreathed in the pink flowers of spring”; ditto Portman, who is the face of Miss Dior. The novel is also set in a European city that bears close resemblance to Paris, where both Portman and Cusk live.

In a recent interview with , Cusk claimed to be bewildered by the noise preceding Life of M, though she wouldn’t be drawn on whether she had conversations with Portman about it, or if rumours of the actor’s displeasure are true. Whether M is based on a real celebrity “is really not the point”, she said, adding: “It’s a work of fiction, and I guess explaining how something is created, because it’s always created out of a mishmash of real things and invented schemes and personal material – once you start to pull on that thread, it would unravel the whole thing.”

Such coyness is to be expected but it is also disingenuous. The mystery of M has proved an effective marketing ploy, garnering copious articles in major titles including , the and long before it has hit the shelves. The hope from Cusk and her publisher will be that those column inches translate into sales. It’s with a drop more honesty that Cusk admits in the same interview that using a famous person as a fictional character is, for her, a “kind of game, and the rules of the game are that the actor and the narrator are both real”.

So has Cusk crossed a line in committing what some are calling a “literary assassination” – or is all fair in love and literature? Graham Greene famously spoke of “the splinter of ice in the heart of a writer”, referring to the cold-bloodedness with which the author must approach those whose lives they plunder.

Autofiction can be especially challenging as it blithely wrong-foots readers who are trying to ascertain what is real and what is not – which, of course, is part of the allure. Among those who have used it to exceptional effect are the French writer Annie Ernaux, the Oranges Are Not the Only Fruit author Jeanette Winterson and Olivia Laing, whose book combined strands of her own life with that of the punk poet Kathy Acker.

Authors defend their approach by observing that truth is a slippery concept. Fiction cannot help but contain a seam of reality as writers look to their own experience and those of others for inspiration. Meanwhile, rare is the memoir that can claim to be objectively truthful, since memory is subjective – and, anyway, aren’t the best memoirs nearly always the ones that don’t worry about hurting other people’s feelings?

In depicting its protagonist’s hollow life, Life of M takes no prisoners. M is uncaring, shallow, condescending and “skilled at giving the appearance of paying attention”. She is surprised, at one point, to meet a young woman in a bookshop whose face is almost a mirror image of her own and who radiates lightness and contentment. M finds it “mesmerising to see it produced so naturally, from mere life”.

Then she notices the woman’s “unexpectedly full hips and sturdy legs beneath her slender upper body”. Cusk could have left it at that but then she delivers the uppercut. “For the rest of the day, M thought about the full hips and the pair of sturdy legs that would have altered her destiny.”

Elsewhere, we see how M moves through the world in a frictionless manner, keeping civilians at arm’s length. The consequence of this is a series of wilfully overlong descriptions of the perfectly designed and curated spaces in which M finds herself, from the inside of a limousine to a luxury spa to a meeting room where she and her staff are assembling to discuss her next project. Cusk seems to be making a solemn point about the airlessness and unreality that comes with perfection, but that doesn’t stop these passages from being unforgivably boring.

If Portman really is furious at Cusk’s alleged depiction of her, you wouldn’t blame her. But the actor can take solace in the fact that the book contains no concrete intel on her or her life. What Cusk gives us is projection and extrapolation. As the reader, the sense you get is that you are learning more about Cusk than you are about Portman. As well as having a jaundiced view of modern celebrity – don’t we all? – she conveys a clear sense of her own moral superiority. Disdain permeates this book, and it’s not all coming from M.

Of course, Cusk makes clear from the start that she is telling an untrue story. It’s the ultimate get-out clause, reiterated in the final pages. When M says to the writer that the portrait of her is hurtful, the writer replies: “It isn’t you… It isn’t anyone.” Cusk has long been a master of keeping the reader on a knife-edge as they search for the line between reality and make-believe, but in Life of M, her insistence that nothing is real – all the while leaning on the biographical details of a famous actor – would seem a case of the author protesting too much.

Near the end of the book, Cusk allows herself a digression that seems to underscore what is really going on. It tells of a woman who meets her estranged son, now a bestselling author, at a book launch. Her son has written about his miserable childhood presided over by neglectful parents. It’s with a note of triumph that the author’s mother tells him: “No suffering, no cruelty. No cruelty, no book.”

‘Life of M’ is published by Faber (£18.99)