In the future, your skin never sags. Your clothes never stain or crumple, your eyes and teeth never look dull, and you never have a bad hair day. That is what life is like, at least, for Dr Aki Ross, the entirely computer-generated female lead of a new film, Final Fantasy: the spirits within, made by the Japanese video-game giant Square. We have had talking playthings in Toy Story and velociraptors in Jurassic Park, but Final Fantasy is the first feature film to populate the cinema screen entirely with digital human beings in a digital landscape.

Ross's cute snub nose and perfect hazel irises have been designed and built on computers, and 60,000 individual strands of hair have been implanted in her scalp and independently animated. Her love interest, Captain Gray Edwards, has the absurdly prognathous jaw of Dan Dare, but at least the computer boffins have also lovingly modelled his enlarged pores and permanent five o'clock shadow.

Final Fantasy is not a completely photorealistic vision - its cast still look like characters in a high-class video game. But the lengths to which someone has gone to create virtual actors has raised fears of a dystopian future when human actors are no longer required. Computer people, Tom Hanks bleated recently, might put real actors out of work. Indeed. Stuff the latest actors' strike, moguls five years hence might say, we don't need those overpaid mannequins: we can make some right here on this beige box. We don't need to feed them, and they'll be completely docile. And look, this one's more realistic than Tom Cruise anyway.

Directors could then present anything they can imagine, rather than having to work with the intransigent stuff of the real world. But we are still waiting for a wizard to brew a magic imagination potion that turns derivative directors into visionaries. The virtual landscapes of Final Fantasy, for example, are bog-standard sci-fi cliche - the scorched earth after an invasion by extraterrestrial creatures; a rocky, rosy alien planet with a huge sun - and remind the viewer of nothing so much as the airbrushed jackets of sci-fi novels.

The characters in Final Fantasy currently have major problems with non-verbal communication - emoting is done mainly with lusciously detailed eyebrows and weirdly elastic mouths, and when a character stops speaking, it is as if it has been switched off. All the little unpredictable facial gestures and muscular tics that we value in the features of a human film star are missing. Designing them ab ovo seems likely to remain an intractably huge problem. And you can forget about sexual chemistry between virtual stars, unless some programmer figures out how to build a romantic consciousness into a set of silicon chips.

Another problem is speech. No one has yet worked out how to make a computer read text with anything approaching human inflection. So Square hired Hollywood actors to do the voices. This provides the weird, if entertainingly distracting, experience of seeing a Ben Affleck lookalike speaking with Alec Baldwin's husky drawl; Donald Sutherland's inimitable moans of half-asleep amazement, meanwhile, issue from the pickled face of an old scientist who looks as if he once had a bit part on The Bill.

Most telling is the particular computer technique that the film-makers use to excess: motion capture. In Final Fantasy, not only were the mouths of the real actors filmed and digitised, so that the virtual characters' lips could be synchronised with the sound, but every bit of walking, clambering and jumping was first done in a studio by stuntmen, so that the movements could be pasted wholesale on to digital skeletons. The directors have made people do all this, and then thrown the people away.

As such, some viewers might be forgiven for considering Final Fantasy to be a giant con trick. Whereas the characters of Shrek or, for that matter, South Park, are carefully hand-animated - their movements are designed by people making artistic decisions - the cast of Final Fantasy are merely the pixellated ghosts of human movement. It is a melancholy species of shadow puppet theatre. A milestone in computer graphics, yes, but the future of cinema? Hardly.

Final Fantasy's failure is the result, in fact, of a kind of inferiority complex vis-a-vis the real world. CGI films will doubtless astonish us in years to come, but they will only be interesting films if they abandon aesthetic competition with live-action cinema. After all, we have been building machines that go faster than human beings for about 170 years, and yet we still regularly hold the Olympics and marvel at one person running faster than another. In the cinema, as in the athletics stadium, we want to watch people.

Final Fantasy (PG) is on nationwide release from 10 August

Steven Poole's Trigger Happy: the inner life of videogames is published in paperback by Fourth Estate (£7.99)