A film that brings excesses of political power into sharp focus

It takes a while for ''The Killing Fields'' to build momentum. But the action becomes elemental and unstoppable once the filmmakers zero in on their real subject - the deadly link between personal arrogance, political control, and the all-consuming nightmare of war.

Readers of the New York Times will recognize the main character, Sydney Schanberg, as a regular columnist on city issues. During the 1970s he covered events related to the Vietnam war in neighboring Cambodia, and - as dramatized in Bruce Robinson's screenplay - broke the first ''unsanitized'' news about the secret American bombing there. His friend and helper was a Cambodian newsman named Dith Pran, who evidently came to the rescue in many tight spots with his native savvy about local language and customs.

Based on their actual experiences, ''The Killing Fields'' begins with a dithering half-hour about the American bombing and the crafty efforts of Schanberg and Pran to ferret out the truth. It's an involving situation, but the incidents are oddly scattered and the characters are hard to grasp at first. Perhaps too much film was left on the cutting-room floor - a miscalculation that would explain later lapses, too, such as the near-nonexistence of a character played by South African playwright Athol Fugard.

After the bombing episode is wrapped up, however, the movie takes on new life. Cambodia's civil war heats up as the rebel Khmer Rouge army advances on the capitol - gunning for everyone who doesn't share its wild-eyed vision of revolution at any cost. As battle rages and corpses pile up around him, Schanberg finds himself in a split situation: As a foreign journalist he benefits from whatever protection the West can still offer in Phnom Penh, yet lives in constant danger that some overzealous yokel will gun him down where he stands.

Hoping to work as long as possible, Schanberg stays in the middle of the fray with Pran at his side. This portion of ''The Killing Fields'' is brilliantly assembled, as the journalists breathlessly fake and improvise their way through one outlandish situation after another. I remember no film that so vividly portrays the bitter contrast between human dignity and bloody wartime chaos, both of which are evoked through leanly constructed yet unstintingly explicit details.

The next segment offers a seeming lull that masks more tension to come. Schanberg and Pran are holed up with other journalists in a secure Western embassy, when suddenly all Cambodians are evicted. An effort to pass Pran off as American leads to the movie's biggest surprise - the disappearance of Schanberg from the main story, which now focuses on Pran's struggle to survive in postrevolutionary Cambodia as the country is stupidly and mercilessly strangled by the utopian morons who have seized control.

''The Killing Fields'' has problems in addition to its hesitant beginning. Its structure has more raw power than internal logic; many characters are whisked away before coming fully to life; and the climaxes drip with conventional movie sentiment. Language is also a barrier - some important lines are difficult to decode in the accented English of the Cambodian costar.

But the plus side outweighs all this. The action sequences are gripping, if harrowing. The cinematographer, Chris Menges, captures the beauty of a place and the horror of a time in deftly composed images, combining sensitivity and color without lapsing into the merely shocking or exotic.

The performances are expertly crafted, too. Sam Waterston couples a keen intelligence with surprising vulnerability as Schanberg, and in the Dith Pran role, Dr. Haing S. Ngor is a revelation - compensating for his diction problems with a stunningly visual portrayal that develops this key character from an unknown factor (in the weak early scenes) to a fully rounded human being of heroic proportions.

An extra measure of praise goes to producer David Puttnam and director Roland Joffe, a very promising newcomer, for the thoughtful undercurrents that deepen the film's impact. Time and again, in various ways, they let us know that ''The Killing Fields'' is no mere true-life melodrama, but an essay on the profound dangers of overweening political power. It's a very long way from the ''secret'' bombing (secret from its victims?) to the murderous discipline and wretched ''reeducation camps'' of the Khmer Rouge fanatics - yet the movie insists that both, like other excesses of right and left worldwide, are the offspring of insolent pride married to blind military might.

Schanberg and Pran survived, and the film celebrates their bravery and cleverness. By recalling our attention to Southeast Asia-related events and attitudes that are still chillingly close to us in time, however, the film also carries a valuable warning message.