Does not compute. Does not...

Comedy? Character development? Why try to extend Arnie's range when he's so good at doing robots.

He will erase your past to protect your future. The poster copy for Eraser is rather promising, raising hopes of a return to the science fiction that is Arnold Schwarzenegger's natural territory, and even hinting at a revisiting of the twin robotic peaks of Arnie's career, the Terminator films. No such luck. Eraser is the first script by Tony Puryear (he had help from Walon Green), billed as "The first African-American to write a major studio action film". He isn't the first, though, to set a thriller in the Federal Witness Protection Program, any more than he will be the last. His hero, John Kruger, is an elite marshal who will go to any length to protect the witnesses in his charge.

If Schwarzenegger made his debut in 1970, with Hercules in New York, and this is the centenary year of the cinema, then he has been making films for over a quarter of the time that there have been films to make. And still when you first see him in a film the question that springs to mind is: who are you and what do you want? As Arnie nears his own half century, he flaunts the big bad bod a little less, and his hair has a dyed deadness, but he is essentially no more implausible than he has ever been.

It's routine to compare Arnie with the action heroes who have wanted to replace him (Lundgren, Van Damme, Seagal, even the oddly fatherly Chuck Norris), but it's much easier to understand what the wannabes want than the man mountain himself. Comparisons with Sylvester Stallone are misleading because Stallone is so full of obvious tensions, though there is nothing in his resume to match the Terminator. Sly is short, he has a flaky mum, he writes and directs, he wants to be loved and make people laugh. Where Stallone is driven, Schwarzenegger seems only programmed.

He wanted to appear in comedies, for instance, rather than to make people laugh. It was a matter of extending the franchise, not changing the technique. And even after Twins and Kindergarten Cop, Schwarzenegger treats a comedy line as if it was a girder to be swung or hurled. The script of Eraser, which isn't a comedy but tries at times to be lively, gives him several styles of android repartee to try. Sometimes the lines are just too long: when someone asks him "Don't you ever get tired of baby-sitting scum?", he shoots back, swift as a heat-seeking missile on a cold morning, "Yes, but in your case I'll make an exception." Too many words! Under James Cameron's guidance, Schwarzenegger worked his way up from "I'll be back" to "Hasta la vista", but really that's about the limit of it.

Then there's the time he surges in at the last minute to rescue this witness who's threatened by... oh just about everybody. She says: "You're late" and he says "traffic". Now this was a joke joke in The Player, mocking the empty bravado of action movie dialogue. Still, someone has had the sense to cut it down from its original unwieldy length, as used in Altman's film, first by Julia Roberts and Bruce Willis, and then by Greta Scacchi and Tim Robbins: "You're late" / "Traffic was a bitch."

The only joke that Schwarzenegger brings off is an impressive three syllables long but, more importantly, it happens at the traditional moment for humour in his movies, immediately after he's killed something for somebody. He kills an alligator and snarls, "You're luggage."

In most films, the hero undergoes some sort of change between the beginning and end of the story - in fact Hollywood movies are riddled with personal growth. But how do you change Arnie? There's a gesture towards this in the script - it would be a bit much to suggest that John Kruger learns to trust, to explore his non-paranoid side, but he does at least break his lifelong rule of working alone. It's even possible that he forms a relationship with the witness, Lee (Vanessa Williams), though it's hard to tell. He has an oddly New Age speech, early on, which might be meant to be charming, about "what you are in here - no one can take that away from you". He's pointing at his sternum at the time. Is that where he stores his chip?

Vanessa Williams is a successful singer and ex-Miss America who clearly hopes that Eraser will do for her what The Bodyguard did for Whitney Houston except... what did The Bodyguard do for Houston? Her character certainly flirts with the marshal, finding fault with the name he's chosen for her new identity: "Do you think of me as a Debra?" All he can do is put his head on one side to look roguish, raise a Cyborg eyebrow and intone, "I could have chosen Debbie! Deb!"

Co-starring with Schwarzenegger, by a cruel stroke of casting, is a spry and spruce James Caan. You'd expect Caan to have the edge in activities such as talking, walking, smiling, making gestures, but even in action sequences he shows Arnie up. You know what he's feeling when he looks at the knife that has appeared in his shoulder and says "I can't believe you nailed me with this cheap piece of mail order shit." Schwarzenegger, by contrast, has a scene where his hand is impaled on a drill bed and he must pull himself free to prevent bad men with futuristic guns from shooting Lee (they've already had her heart, pulsating like a bullfrog, in their CAT-scanning sights). And still we can't decide from his face whether he's pretending to feel pain, or pretending not to. His vulnerability and his invulnerability are equally empty categories.

The director, Charles Russell, has a background in horror (the third Nightmare on Elm Street, the remade Blob) and special-effects heavy comedy, as in the Jim Carrey vehicle The Mask. He's shrewd enough to know that audiences of action films don't want to watch dull computers, so the state- of-the-art data facility where the dark secrets are stored seems to have been modelled on a Wurlitzer jukebox from the 1950s. He does come up with one fine action sequence about halfway through the film - a crucial juncture in thriller dynamics - in which the hero sabotages a plane to get out of trouble, and has to face the consequences. The moment where he has to disentangle the strings of the parachute that is wrapped around him before the up-rushing ground ends the argument will strike a chord with anyone who has ever tried to sort out a child's tangled marionette.

n On general release from Friday