A few weeks ago, some of us at NEWSWEEK got an advance screening of "The Polar Express" with the best possible host--Bob Zemeckis, the director of the $165 million film based on the popular children's book. Before we saw the movie, Zemeckis showed us the creation process. Real actors (notably Tom Hanks, who plays five roles) were festooned with costumes and sensors so that all their motions, down to the slightest facial twitch, could be detected by special cameras, stored digitally and mapped to computer-drawn characters. Though "Polar Express" is clearly a computer-graphic (CG) animated film, Zemeckis resisted that description, insisting that his "performance capture" methodology makes it something more akin to a realistic movie. He clearly believed he'd cracked the code of humanizing the CG human, a long-discussed hurdle in the field.

Lurking in the background of the discussion was "The Incredibles," the newest effort from Pixar, which hit the studios five days before "Polar's" debut last weekend. Since humans in earlier Pixar films were seen as less convincing than the brilliantly rendered toys, bugs and fish that had headlined the earlier hits, many were wondering how Pixar would fare with a film whose protagonists were folks like us.

As a kid, I used to watch "Popeye" cartoons every day, and not once did I question whether the animators had "sold" me on believing that this balloon-jawed spinach gulper was a real person. On the other hand, when Robin Williams immersed himself in makeup and prostheses to portray Popeye in the eponymous movie, I didn't buy it for a minute. Therein lies the paradox--the closer you get to the human form that our eyes see every day, the more challenging it is.

Interestingly, both Zemeckis and Pixar's Rick Sayre, the supervising technical director on "Incredibles," agree that performance is key to vivifying a CG-generated human. But when Zemeckis talks about performance, he means something that Tom Hanks does with 165 sensors glued to his face. (Though stuff like the eyes and tongue aren't "captured.") Sayre defines performance as the way a CG character is scripted, framed, moved and lit. This is a view shared by the team at DreamWorks. "We don't do any motion capture," says "Shrek" producer Aaron Warner. "Our animators are actors in and of themselves."

Has Zemeckis's bet paid off? While there is much to visually admire in "Polar Express," the people have a bizarre wax-museum quality. As Variety put it, "the three dead-eyed tykes who lead the action here resemble nothing so much as Stepford children, giving the film at times a creepy feel." Meanwhile, Pixar chose to portray its "Incredibles" superheroes as stylized, comic-booky figures that sometimes defy physics but nonetheless move and emote in a humanlike fashion. Critics loved it, and it was all done from scratch. "You can't get a performance with motion capture," says Sayre. "It's a mistaken notion."

In short, there are logical roles for both humans and computers. Zemeckis talks about a future where performance capture will be used even in non-animated films; auteurs will draw on prefab performances by our best actors--tours de force fast-frozen, thawed out and fitted with new digital skins. But why bother when there are plenty of flesh-and-blood thespians who can do wonderful, spontaneous work? Likewise, if you're creating an original form of reality by computer-animating a movie, why chain yourself to the gravity-bound machinations of a plain old person? If we've learned anything in the past half century of digital technology, it's that some stuff is better done with computers. And other stuff not.