Attack of the avatar!

They look real, they sound real... these days, they even have souls. Computer-generated film stars are here with a vengeance. Is the humble actor doomed, asks Jonathan Romney. Or is this the beginning of a beautiful new friendship?

At some point in the 1990s, the film world began to entertain an obscure terror - or, alternatively, a strange utopian dream. It began to believe that in the near future, screen actors - of the old, fallible flesh-and-blood variety - might become obsolete, or at least that they would need to come up with some seriously impressive new tricks if they were to justify their continuing presence (and, in some cases, their stratospheric fees). What threatened to eclipse human actors was the advent of computer-generated ones, "synthespians", or to use a briefly modish term, "avatars". Given that computer-generated imagery (CGI) could create plausible dinosaurs, hordes of invading aliens and shape-shifting metal men like the scene-stealer in Terminator 2, it surely couldn't be too long before convincing photorealistic humans could be created. When that day came, then the exorbitantly paid Schwarzeneggers and Cruises could look to their laurels.

We can scoff and say it will never happen - yet in a sense, it already has, although the big stars have so far been less affected than the bit players. In 1996, James Cameron announced plans for a $100m project entitled Avatar, billed as "the first ever film to include fully CGI actors". Trade paper Screen International speculated that the film would "rewrite the rules for compensating Hollywood talent"; the Screen Actors Guild, it reported, was concerned that synthespians would take humans' jobs, or that actors would be inadequately paid for letting themselves be scanned for digital use. Contracts for films like Avatar would have to take into account "a new form of performance". Avatar never materialised, although plenty has happened in the last decade to cause the SAG real worry. Hollywood's traditional cast of thousands has now routinely been replaced by casts of millions: millions of pixels, that is, with digitally generated humans mustered as extras for battle scenes on a previously unthinkable scale, in films such as Troy, The Lord of the Rings trilogy and the second tranche of Star Wars episodes.

But could synthespians put traditional star power out of business? It never really seemed a possibility, until now. But we may be witnessing something of a crisis in the Hollywood star system. Earlier this month, the US box-office chart was topped by The Incredibles, the Pixar digital animation about a family of cartoon superheroes; it made $70.5m in its opening weekend. The following weekend, it took over twice as much as the opening gross ($23m) of the number two film, Robert Zemeckis's The Polar Express, a $165m action-adventure vehicle for one of Hollywood's most bankable and durable stars, Tom Hanks. If a Hanks Christmas blockbuster can be thrashed by a gang of rubbery-looking 3-D cartoons, what does that say about the current status of human stars?

In fact, The Incredibles/Polar Express playoff is more complex than that suggests. If the Pixar animation has beaten Hanks, it's not because viewers are turning away from the human factor on screen, but rather because Mr Incredible and his family come across as more humanly charismatic than Hanks does in The Polar Express. Whatever considerable charm Hanks may have brought to his biggest screen successes is notably absent from the overpowering, weirdly alienating monument of kitsch that is The Polar Express. But one suspects that Zemeckis's film has underperformed neither because of, nor despite of Hanks, but rather because its viewers are vexed by this question: in what sense is Tom Hanks actually in the film at all? You'd have to say that Hanks "stars" in The Polar Express only in a very peculiar, indirect sense: he's only virtually in it.

Hanks plays - the term requires some redefining here - five characters, thanks to a new method called "performance capture". Based on Chris van Allsburg's illustrated children's book, The Polar Express is an elaborate hybrid of digital animation and live action, a blend of seasonal sentiment and breakneck action. In this featherweight parable about the power of the imagination (imagination that, remember, cost $165m to invoke), a nameless young boy (played by Hanks), who no longer believes in Santa Claus, is whisked away on a magical train ride by a jovial conductor (played by Hanks). En route to the North Pole, the boy meets a ghostly hobo (Hanks) before encountering Santa himself (Hanks) and returning home to his dad (Hanks). There is also a rather terrifying Scrooge puppet, again played by Hanks.

But "played by" in what sense? As the term "performance capture" suggests, the essence of Hanks has been digitally distilled and infused into several animated characters who more or less resemble him. The technology, devised by Sony Pictures Imageworks, is effectively a refinement of a method known as "motion capture", in which a number of reflecting points attached to an actor's body allow his or her physical movements to be logged and then applied to a digitally generated figure on screen. Most famously, motion capture is the process that helped to generate the eerily lifelike Gollum played by Andy Serkis in Peter Jackson's The Lord of the Rings films: it is Serkis's original physical performance, the reality of his gymnastic contortions, that provides the virtual musculature that makes Gollum appear real. A similar process - though no doubt in a more sophisticated refinement - will turn Serkis into the rather larger protagonist of Jackson's King Kong, scheduled for Christmas 2005, and will allow him to get his furry paws round a human-sized Naomi Watts.

The "performance capture" used in The Polar Express is slightly different, in that it aims to capture not just the body but also the soul: to map not just physical movements but also facial expressions. It involves some 150 small reflectors, or "jewels", applied to an actor's face, tracking expressions that can be mapped on to characters created in the computer. In The Polar Express, this results in some bizarre effects.

Two characters are visibly derived from Tom Hanks, in that they share his facial features, however stylised: the moustachioed conductor and the faintly Tom Waits-like phantom hobo. As for the principal child, billed simply as Hero Boy, his facial features were scanned from a child chosen because he resembled Hanks as a boy, while the character's voice is actually provided by a young actor, Daryl Sabara.

If Hanks "plays" Hero Boy at all, it's insofar as his facial performance, "captured" by the technology, has been used to animate the boy's face. If you scrutinise Hero Boy's features for traces of Hanks, then you start to recognise little giveaway signs: a quizzical pursing of the lips, a puckish raised eyebrow, an owlish puffing of the cheeks. But if Hanks is not speaking the role, only "animating" the Boy from within - "animating" in the sense of imbuing him with soul - then we really do have to redefine what we mean by acting, and by on-screen presence. A "new form of performance" indeed.

As Peter Biskind explains performance capture in a recent Vanity Fair article, the process allows Hanks "to almost literally get under the skin of another performer" - an analogy eerily suggestive of demonic possession. The film's oddly android-like child characters do indeed seem to be harbouring other presences under their digital skins: real children's faces have been scanned in, then "animated" by the performances of adult actors, and then dubbed with the voices of other actors again. Ken Ralston, the film's senior visual effects supervisor, has stressed: "Every kid in this movie is played by an adult." And Robert Zemeckis insists that his characters are not animated but genuinely acted: he states in the film's press notes: "The expressions are all done by the human actors. The computer does not create the performance; the humans do. The computer just takes the performance and wraps a cinematic skin around it." The Polar Express's bizarre human-digital hybrid has certainly confused viewers, and the industry too: Warner Bros had to submit the film to the Academy to determine whether or not it is eligible for the animated feature category in the forthcoming Oscars, or must be classified as live action. As for US critics, they have found The Polar Express both uncanny and unpalatable. Roger Ebert was one of the few to use the word "creepy" in a positive sense, admiring the film's "shivery tone" and praising the characters for a "simplified and underlined reality that makes them visually magnetic". Most other reviewers recoiled. Variety called the film "emotionally frigid", comparing its "dead-eyed tykes" to "Stepford Children", while Rolling Stone found them "almost spooky in an Invasion of the Bodysnatchers kind of way". Most damningly, Manohla Dargis in The New York Times complained: "None of the humans have the countless discrete fluctuations, the pulsing, smiling, twitching aliveness that can make [Hanks] such a pleasure to watch on screen." If Hanks isn't truly "in" this film that he's not actually in, then where actually is he - and what's the point of him being there? Dargis also makes a telling comparison. On one side are the rich achievements of Pixar's animators, who brilliantly infuse their simulacra with character. (After all, the plastic-faced cowboy Woody, voiced by Hanks in Pixar's 1995 Toy Story, is far more expressive that any of the Polar Express humans.) On the other are film-makers such as George Lucas (and presumably Zemeckis too) "who seem intent on dispensing with messy annoyances like human actors even while they meticulously create a vacuum-sealed simulacrum of the world". Paradoxically, while Hollywood technocrats dream of dispensing with actors and all their attendant inconveniences - egos, fees, agents, human error - they also want to capture the ineffable quality of personality or performance, albeit in an ideal, abstract, antiseptic zone detached from time, space or the limitations of human presence.

Enthusing about the Pandora's box opened up by performance capture, Peter Biskind's Vanity Fair article rehearses some long-cherished fantasies of digital cinema, among them the idea that actors will soon be able to scan their likenesses into computers and play younger versions of themselves. His image of Brad Pitt at 50 playing himself at 20 surely suggests a nightmare prospect of an undying battalion of digital Dorian Grays keeping new talent out of the business.

Biskind also invokes "that holy grail of CGI: a reborn Marilyn Monroe, bigger, better and more fetching than ever". With performance capture, he suggests, you could even build a composite of Monroe's performances and have Halle Berry reanimate her from within in a new film. The logical outcome, you suspect, would be a practice of revising classic film performances in accordance with today's tastes, just as old hit records get remixed to the season's modish beats: instead of just remaking James Stewart movies with Adam Sandler in the lead, why not drape Stewart's digital skin on the virtual bones of a Sandler performance? The result surely couldn't be any worse.

But rather than simply raising the screen dead, cinema has in recent years aspired to create living, breathing digital forms of its own. So far, it has worked brilliantly when those forms are clearly artificial and stylised and follow their own physical rules, as in The Incredibles. But the photorealistic humanoid that can stand in for flesh and blood still remains a problematic goal. In Andrew Niccol's 2002 film Simone, a self-conscious rewrite of the Pygmalion myth for the pixel age, Al Pacino plays a Hollywood director, frustrated by dealing with neurotic actors, who digitally creates the perfect, perfectly biddable, starlet. Simone (from "Simulation One") is so seductive that she not only convinces everyone that she's real, she also becomes a genuine screen goddess.

Niccol's film never rises above the level of a sour joke, but the world has seen real Simones. In 2001, the computer game spin-off Final Fantasy: The Spirits Within was the first all-digital animation to feature photorealistic human characters, notably heroine Dr Aki Ross (the film's undisputed USP, its male characters receiving little publicity). Given the intensely eroticised sales appeal of Eidos's Tomb Raider heroine Lara Croft, Aki was surprisingly sexless, her blandly hyper-detailed perfections lacking the raunchiness of Lara's relatively rudimentary geometric planes. But Aki won hands down when it came to detail: her defining mark of realism was a coiffure composed of 80,000 hairs, which took considerable computing power to generate. Aki didn't set the world alight: perhaps her target fanbase of adolescent boys just wasn't that interested in virtual hairdressing. Still, the search for CG cheesecake persists: a Miss Digital World virtual beauty pageant is currently being held, with entrants competing on the counts of both sex appeal and verisimilitude. Contenders include Kaya, made in Sao Paolo and composed of 48,200 "polygons", or geometric units of information, and the German-"born" Sophie Winter, a mere 20,000 polygons, but whose assets, her makers claim, are her eyes - "they are really the window to the soul of a character". Who knows, perhaps one of them will be discovered and cast opposite that virtual 20-year-old Brad Pitt?

By definition, there is little of the organic about CGI's virtual creatures, yet they seem to be conceived as composites of qualities that exist elsewhere, just as Hanks's Hero Boy is actually derived from three different living people. The one genuine insight of Simone is the way that its supposedly incomparable diva can be analysed into her constituent parts: we are told she has "the voice of a young Jane Fonda, the body of Sophia Loren, the grace of Grace Kelly and the face of Audrey Hepburn combined with an angel". One of the film's few genuinely comic moments has Pacino "mixing" Simone from the pre-sampled qualities of other stars: "Less Streep, more Bacall". We can mock and say that chimeras like Simone could never happen - that human presence is the guarantee of on-screen authenticity, that nothing is more lifelike than the living. But that's not so: indeed, the deadness that strikes us on watching The Polar Express stems from our sense that it carries traces of the human that perhaps shouldn't really be there. The film would be far less discomforting if Zemeckis hadn't instilled it with Hanks's virtual presence, but simply created autonomous digital humanoids. Instead, his artificial interface between living actors and digitals strikes us as oddly morbid.

A similar case is the recent digital feature Sky Captain and the World of Tomorrow, a futuristic pastiche of 1930s sci-fi, which inserts a scattering of live actors into digitally generated settings. Theoretically, it was the use of human leads, Jude Law and Gwyneth Paltrow, that should have brought a charge of muscular energy to Kerry Conran's otherwise pallid fantasy. Instead, these stars' presence accentuates the film's airless quality. Their flat, stylised performances make it feel as if the actors have had their souls sucked out by the surrounding artifice, effectively becoming digital figures themselves: Law and Paltrow seem two-dimensional in a three-dimensional world.

More disturbingly - and here's where Sky Captain may prove to be of some slim historical interest - the film boasted one performance that felt uncannily larger than life. Its evil scientist Totenkopf - a digital melding of Dr Mabuse and the Wizard of Oz - spoke from beyond the grave as a disembodied face, textured like a flickering early television image. Totenkopf was "played" by the late Sir Laurence Olivier, as he looked in the 1930s: an apparition sampled from the actor's films of the period. (Even more bizarrely, Olivier was routinely listed among the film's cast, as if alive - just as Monroe no doubt will be when she's digitally spliced with Halle Berry.)

Olivier's phantasmal cameo would be of anecdotal interest only if it didn't set a disturbing precedent, and if he didn't so decisively upstage his living co-stars. That itself is one reason why Hollywood is so intrigued by digital talent. We constantly hear the lament that stars are no longer as distinctive as they were, that there are no new Mitchums or Dietrichs, even Hankses or Cruises, in sight. At a time when fame itself is devalued by inflation, in a market flooded with ephemeral, interchangeable celebs, it may be that the very concept of the star needs to be comprehensively rethought. Hollywood cast lists are increasingly filled with names whose distinguishing qualities are so elusive that there would be little point scanning them for cryogenic-style digital preservation: why capture for posterity the co-ordinates of an Ashton Kutcher or a Heath Ledger, a Tara Reid or a Kate Bosworth? In the multimillion-dollar echelons of Hollywood acting, at least, performance may never again be entirely separate from the digital realm: stars must henceforth learn to work with CG supporting players, and be prepared to be at least partly digitised themselves. The organic will have to fuse with the electronic, and the most interesting performances will be those which respond most ingeniously to that challenge. So we should at least give credit to Hanks and to Zemeckis for undertaking the bizarre adventure of The Polar Express.

As digital Christmas blockbusters go, however, the really intriguing one may yet prove to be next month's Lemony Snicket's Series of Unfortunate Events, which looks from the trailer as if it will be every bit as thoroughly digital as anything mentioned above. Yet its selling point - its irreducible human factor - comes in the shape of Jim Carrey who, like Hanks, appears in several roles, but heavily disguised in traditional latex chins and comedy wigs. Carrey harks back to an older tradition of the protean character actor, a disguise artist in the line of Peter Sellers, Alec Guinness, or indeed - given Carrey's often torturous physicality - Lon Chaney. Instantly recognisable, Carrey's screen presence is nevertheless as malleable, protean and elastic as any conglomeration of pixels and polygons could ever hope to be. As cinema's digital age continues, it may be expressionist players like Carrey who, in all their cartoonish quality, prove to be the last repository of traditional unprogrammed and unsynthesised star quality.

'The Polar Express' is released on Friday, 'Lemony Snicket's Series of Unfortunate Events' on 17 December