I can really only go on supposition, but I do tend to assume that the life of the super-beautiful movie star contains less in the way of futility and failure as those of we shabby mortals. There is one thing the gilded screen god or goddess will never know, and that's what it's like to be ugly. In fact, scratch ugly – even the experience of being blandly average is intrinsically beyond them. Which, for even the most gifted actor, is going to be a hurdle in a biopic of someone demonstrably plain.

On this supposition is founded the misgivings I've always had about Howl, the study of poet Allen Ginsberg at the time of his obscenity trial in 1957 which will debut at next month's Sundance festival. Because, as shown in the still posted at the House Next Door, Howl sees Ginsberg being played by James Franco: yep, the famously homely Allen Ginsberg is portrayed by possibly the most beautiful actor in Hollywood. And they've tried, you can tell they really have, with the haircut and the rest of it. But with all appropriate caveats in place about not judging a film I haven't seen yet, I will confess to having a chronically hard time buying into Franco's Ginsberg – and I say this as someone who likes James Franco – as anything other than a hopeful pair of glasses.

Of course a healthy suspension of disbelief is a must for any film lover, and mine's a marvel – a deeply suggestible nature that's been honed over years to the extent I can believe in Lotte Reiniger silhouette puppets and the films of Val Kilmer almost as readily as I can the laptop screen before me now. And yet the biopic fitted out with the excessively perfect star is, for me at least, a near-certain deal-breaker. The sleight of hand at the heart of cinema history – the unreal splendour of most screen performers – is at its very shakiest when Google Images can instantly remind you of the lank, spotty, asymmetrical reality they're meant to be capturing.

And what's doubly strange to me is that, with notable exceptions such as Nicole Kidman and her proboscis in The Hours, this particular syndrome seems to affect male leads as much – if not more than – women (maybe because, the culture being what it is, women deemed deserving of the biopic treatment are often attractive to begin with). Howl alone falls for it not once but twice – casting Mad Men's Jon Hamm as unchiselled defence lawyer Jake Ehrlich. And I can tell you, those few straight women and gay men I know who don't on some level want James Franco, assuredly want Jon Hamm. And if they don't, then they do want Johnny Depp, whose fine performances as various real people have always been subverted by the implausibility of a man who looks like Johnny Depp meeting the same fate as, say, hapless coke smuggler George Jung in Blow. And this is, I feel, the moment at which it becomes necessary to reference Jared Leto's appearance as John Lennon's killer Mark Chapman in the ill-fated Chapter 27.

The point, of course, isn't that an actor need look like their subject in order to play them. Much as the biopics you like most are often about people you aren't especially fond of (or were even previously interested in), so it's a given that there's more to the weird, occultish practice of conjuring up a real subject than mere looky-likeyness. Witness genre staple Michael Sheen's non-resemblance to Tony Blair or Kenneth Williams, or the lack of similarity between Andy Serkis and Ian Brady or, indeed, Ian Dury – his part in the forthcoming Sex & Drugs & Rock & Roll, from which photographer Sarah Lee's on-set stills already feel a good deal more authentic than Howl.

And I'm willing to concede that this may just be a personal blind spot. But I can't help believing that if the biopic is about mining truth from the subject's soul, then it's that much harder when the schmuck in question has the face of a schmuck and the film star playing them looks like a film star. For most of us, our physical mediocrity is central to our identity, as it surely was for Ginsberg. And while it's clearly possible to be (to quote the sage Derek Zoolander) really, really ridiculously good-looking, and still be a gifted actor, when the role calls for the ordinary, the excessively handsome male lead can only skim the surface. Skim with great bone structure, yes, but skim nonetheless.