The increasing use of CG and green screen shows it ain't what you've got, but how you use it, says Lee Marshall.

In one of this autumn's most hotly anticipated releases - Robert Zemeckis' adaptation of the Old English epic poem Beowulf - Angelina Jolie plays a sexy seductress who attempts to distract our hero from his mission and win him over to the dark side. To emphasise her 'bad girl' credentials, she wears her hair in a long plait that swings around like a scorpion's tail. The usual CGI smoke and mirrors, you might think.

But one glance at the recently released trailer reveals that it's not just Jolie's Lara Croft-like pony tail that has been digitally enhanced. It's everything about her. She has fuller hips, plumper lips and a tan so golden you could prospect for nuggets in it. All this without a single visit to the solarium, the gym, the plastic surgeon - or even the make-up department.

During the shoot, Jolie wore a CGI bodysuit peppered with sensors (see above)- rather like the motion-capture gear worn by Andy Serkis as Gollum in The Lord Of The Rings trilogy, or by Bill Nighy when he played the tentacular, barnacle-encrusted pirate captain Davy Jones in the second and third Pirates Of The Caribbean films.

In Beowulf, however, only one main character - the monster Grendel, played by Crispin Glover - has been Gollumised. The others - Ray Winstone as Beowulf, Anthony Hopkins as King Hrothgar and Robin Wright Penn as Queen Wealhtheow - are recognisably themselves... except for one or two important tweaks.

Winstone, for example, is more muscular and looks about half his age. Wright Penn is made to seem both older and younger in the course of the film. By taking an actor's electronic co-ordinates and squeezing them, inflating them, ageing them, rejuvenating them or covering them with tentacles, today's major CGI workshops - whether it be Industrial Light & Magic, which worked on Pirates Of The Caribbean, or Sony Pictures Imageworks, which is behind Beowulf - can turn them into 'synthespians': distorted echoes of themselves, like a game of Chinese whispers played with people, rather than words.

But why go to all that trouble of putting Angelina in computer pyjamas (a Nighy coinage) if the result is a photo-realist Angelina doll'

Well, for one, when it's not really you it doesn't seem so bad to show a bit of flesh - actually quite a lot of flesh, if the trailer is anything to go by. In effect, Ms Jolie has become her own body double. So that's one crew member axed, along with most of the costume and make-up department.

The cinematographer doesn't have a lot to do either. The actors just put on their computer pyjamas and act (everyone involved in any motion-capture film is terrifically keen to let us know they really are acting - though the Academy has not yet seen fit to honour any mo-cap performance with so much as a nomination).

The information about the actors' movements and expressions is relayed electronically, without ever passing through a camera (even greenscreen, as used in Sin City or 300, becomes unnecessary at this point). Then the 3D animators go to work.

The neat, and alarming, thing about this level of performance capture is that you can re-run any scene from any angle you like, Matrix-style. You can choose, within certain limits, to dolly through or zoom in on a detail you may have missed.

So there are no worries about getting the shot right first time - you can just reshoot it later, without actually having to reshoot. In fact, the film could be 'directed' by a competent editor ('plus ca change,' most competent editors would reply).

In the end, it all comes down to that crusty old adage about it not being what you've got, but how you use it. Why is it that a film such as 2001: A Space Odyssey, with its scale models and primitive Go-Motion technique, still looks fresh today, while an all-greenscreen film such as Sky Captain And The World Of Tomorrow is already showing its age' Hint: it's not the technology.