To watch “The Polar Express” in process and onscreen is to begin to understand the promise of the new technology and its potential drawbacks.

There was little that resembled a traditional shoot at a warehouse in Culver City, Calif., where the film was being made in April. In place of a soundstage, there was a domelike structure built of scaffolding that surrounded a playing area roughly 10 feet square. Attached to the scaffolding were several dozen infrared sensors, which could pick up and digitally record the light bounced back by the dozens of small reflectors on Mr. Hanks’s black bodysuit, as well as by the 150 smaller reflectors attached to his facial muscles. With his face dotted by the tiny jewels, as the crew called the reflectors, Mr. Hanks looked like the pincushion man from the “Hellraiser” series. But after a few days of working with the reflectors attached, he said, he no longer noticed them.
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When Mr. Hanks entered the playing space - “the volume,” as Mr. Zemeckis likes to call it - his movements were recorded by a computer as points of light floating in a dark three-dimensional space. Even in this raw form, the connect-the-dots figure moving on the computer monitor was recognizably Mr. Hanks. It walked like him, gestured like him and, most important, crinkled and smiled and frowned like him.

Filmmakers have been able to capture full-body motion for some years using a process called mo-cap, in which a computer scans sensors attached to a performer’s limbs and records the broad outlines of movements. “It’s been around a long time from video games,” Mr. Zemeckis explained. “They put sensors on the athletes for sports games and things like that.”

The great leap of “The Polar Express” came in the ability to capture facial expressions: “When we did the first tests,” Mr. Zemeckis said, “we had Tom do the body acting, and then we put him into a space where he sat in a chair and had to re-act everything from the neck up. I said, ‘You can’t do a movie like this.’ So they went back and were able to figure out how to get both sets of sensors working at the same time. And once we started the movie, the technology kept getting better.”