One of the nice minor things about being at home in the daytime is that you get to lie on the sofa watching crummy biopics on the Hallmark channel; personally, I can never get enough of watching a young Frank Sinatra, who for some reason closely resembles the late Marty Feldman, being screamed at by his mother, the neighbourhood abortionist, "Hat Pin Dolly", that he’ll never amount to anything, you hear, ya lousy schmuck! Or uttering a wail of forewarned dismay as the young Norma Jean Baker takes the towel from her previously brunette hair to reveal a halo of palest yellow, and with trembling lips vows to the reflection in the mirror that now, born again blonde, a life of happiness and true, lasting love awaits her. Just as much fun are those "classy" numbers in which earnest young American writers sit around outside Paris pavement cafés discussing Modernism. "Ah, Ezra, you old fraud! — written any good poems recently?"

Still, perhaps what they are being paid is danger money, because the actor who agrees to play someone of substance — or even another actor, heh heh — may end up looking foolish in a variety of ways. First, there may well be accusations of hypocrisy. Actors scream like hairdressers about the violation of their "civil rights" at the hands of the fascist press’n’paps and the bloodsucking, tabloid-grabbing public every time a poxy little snapshot of them looking less than perfect is published — and then they take large sums of money in order to delve in great depth, over the space of one and a half hours or more, into the lives of Sylvia Plath (Gwyneth Paltrow), Virginia Woolf (Nicole Kidman) Frieda Kahlo (Salma Hayek) or that poor, mad bloke who was good at sums (Russell Crowe) in order to entertain said drooling public. Their sex kicks, their lunacy, their suicides — all is laid bare as these unquiet graves are robbed in glorious wide screen Technicolor by these oh-so-private thesps.

You can understand why people who do such an insubstantial job as acting would jump at the chance to pretend to be something rather more solid, like writers or mathematicians. But they don’t realise how daft they look in comparison, or how silly they sound, as when Kate Winslet said that she was a great fan of Iris Murdoch — though she had only "dipped into" her books! What did she admire about her, then — the way she went nuts? The spectacle of a lightweight mind coming up against a heavyweight subject is one of the more decadent pleasures of modern life, for sure — with typical sycophancy and no apparent irony, Vogue magazine wrote breathlessly of Winslet as Murdoch, Kidman as Woolf and Paltrow as Plath as proof that "Hollywood actresses are lining up for roles that will project their brains as well as their beauty". Excuse me — Hollywood brains?!

But it isn’t just in comparison with assorted literary and mathematical geniuses that modern actors look about as substantial as paper dolls and shadow puppets; when contemporary film stars play bygone film stars — no honour among thieves there, obviously! — they come off just as badly. There is something so flimsy, so half-assed and ill-formed, so downright inappropriate about these posthumous pairings that even the words "Gwyneth Paltrow as Marlene Dietrich" (as is now being rumoured) are enough to make me giggle. So I very much doubt whether I shall be rushing out to see Martin Scorsese’s forthcoming film The Aviator, a life of Howard Hughes in which Gwen Stefani plays Jean Harlow, Kate Beckinsale plays — gulp — Ava Gardner and Cate Blanchett plays Katharine Hepburn.

Now, I’m going to go against type and not take my usual sledgehammer to crack a walnut here. Miss Stefani seems a charming and intelligent young woman who I am sure will be true to Harlow’s sweetness; Miss Blanchett actually seems far too good to play an actress of whom Dorothy Parker said: "Her emotions ran the gamut from A to B and back again." But the words "Kate Beckinsale as Ava Gardner" fill me with the same amusement and amazement as the Paltrow/Dietrich mismatch.

Why should this be the case, I wonder? On the surface, today’s film stars have everything going for them. They are not plucked from soda fountain stools at the age of 16 and pitched headlong on to the casting couch and into the charm school, emerging as screwed-up Stepford starlets. They are better educated, better trained and no longer puppets of an all-powerful studio system which could almost hire them out like hookers or heifers. BUT THE MAGIC IS GONE.

Why? Well, it’s partly a class thing and mostly a sex thing — like most things. Today’s film actresses generally come from middle-class or showbiz families; either way they feel safe — and they look safe, too. There is none of the raw, driving panic, economic at its root, that made every great movie star from Clara Bow to Marilyn Monroe so compelling. Despite this, they flaunt themselves on screen, even revealing intimate parts — and because we know they don ’t need to, and they could easily make a decent living with their clothes on, we despise them in the way that the proletarian prostitute despises bourgeois "amateurs".

In LA Confidential the mighty Kim Basinger — the exception that proves the rule, along with Rachel Weisz and Angelina Jolie; a great film star as well as actress — played a call-girl who resembled Veronica Lake, working in a Hollywood brothel specialising in hiring out star-lookalikes for sex. Autonomous, empowered individuals as they are, some modern actresses who mimic the great stars for money — especially if they get their tits out, as the great stars never had to — are behaving in this long if not noble tradition. But hey, let’s face it, it’s their best shot at immortality; in 50 years’ time, will actresses be queueing up to play Gwyneth Paltrow, Nicole Kidman and Kate Winslet in intimate biopics, as they have exposed others? One almost wishes the answer was yes, because it would so serve these humourless hypocrites right. But one knows, no matter how much they may mimic their betters, that they will always be in the same position as poor Geraldine Dvorak, Garbo’s understudy, of whom it was said: "She has everything that Garbo has — except whatever it is that Garbo has." When you see Kate Beckinsale "as" Ava Gardner, try not to laugh too loud.