"Aw, come on now - you know about my debutante..."

There are two key surprises in the new Edie Sedgwick biopic, Factory Girl.

The second comes right at the end, as the credits start to roll indeed - among all the clearly-real-life characters involved, the blatantly Bob Dylan part played so lamely by Hayden Christiansen is actually listed, disingenuously, as "Folk Singer".

Why so coy?

It's not as if the affair between Dylan and Edie isn't well-documented.

Nor is he portrayed so much as the villain of the piece, of Edie's downfall, compared to Guy Pearce's lip-twitching, eerie Andy Warhol.

I can only guess that Dylan threatened to sue once he'd got wind of just how horribly vacuous Christiansen could be, the "actor" appearing to believe that a shrugged-on leather jacket, a mussed-up mop and the odd dreary drawl add up to enough of an enigma.

The first surprise, on the other hand, is that Sienna can actually act.

Very well indeed, in fact.

Okay, so the role seems oh-so-nattily tailor-made for her - a flimsy, shimmying social gadabout-town, expertly wearing all the chic-est outfits and even more expertly falling out of them at any excuse.

But you do start to care as the starts to fall apart, cast adrift between two (supposedly) powerful presences and the ever-whirling tastes of the times.

Except... the film as a whole doesn't help her.

It's too fractured, too shallow - cutting up abruptly, more Thomas Crown Affair at times than really-radical pop-artistry, and there's just not enough reason to understand why Edie Sedgwick slipped away.

Nor why to truly, madly, deeply care.

Nor is the effect assisted by the too-predictable framework, intercutting flashback scenes with emotion-recollected-in-tranquillity commentaries - the kind of start-at-the-end structure that seems motivated less by artistic merit, the attempt to reflect historical hindsight and its unreliability, than to distance such films from being merely a straight-to-TV docudrama.

The worst of these was the lamentable Cole Porter biopic, De-Lovely, in which the old-Cole-commentates-on-the-younger conceit was nowhere near so clever as it thought, being merely distracting all the while...

So, well, while Factory Girl is a stirring little Sixties tale, hopefully speeding the return of such fashions to the frontline, there's just - like Edie by the end - a little too little flesh around the bones.

When all it really needed, all in all, was another, well, fifteen minutes...?