'Under the Volcano' - a model of how to film a book. Huston and company relate a glum narrative with high excellence

I was skeptical about ''Under the Volcano.'' Many writers and directors have tried to film Malcolm Lowry's novel since it was published in 1947, but none have gotten past the planning stage. And it's easy to see why. The story is fragmented; the mood is gloomy; the hero is drunk most of the time; and the element that justifies all this - Lowry's prose style - is often self-conscious and arty despite the book's vaunted reputation.

How to translate such an eccentric work into screen terms? And would the effort be worth the bother?

To my surprise, filmmaker John Huston and a group of superb actors have accomplished the feat almost brilliantly. Although flawed by incoherence at moments, their version is a model of literary adaptation - intensely dramatic, sharply cinematic, and full of passionate performances. In all, it's quite a turnaround from Huston's last book-inspired effort, the misfired adaptation of Flannery O'Connor's amazing ''Wise Blood.

The biggest challenge presented by ''Under the Volcano'' is its main character: a British consul in Mexico, struggling to find a few decent moments during the last boozy day of his disappointing life. Even such a strong Lowry supporter as critic Stephen Spender admits there is ''some weight'' to the view that a book with such a hero is not ''about normal life'' and needn't concern many readers.

Spender responds that the Consul's addiction is caused by loneliness, surely a legitimate topic for fiction, and is a manifestation of Lowry's deeper subjects: ''the world in these times'' and ''the breakdown of values'' in our century. This is a persuasive argument, but it offers little help to the ''Volcano'' filmmaker, who must transmogrify symbols and symptoms into living, breathing, sympathetic screen images.

Huston and his colleagues meet this challenge in several ways. For one, the director encourages our identification with the Consul by following his actions with long, uninterrupted shots that draw us into his moods and feelings more effectively than standard quick-cutting shots could do.

For another, the screenplay (by Guy Gallo) keeps him at the center of attention by replacing the novel's involved structure with a more directly character-oriented approach.

And just as important, Albert Finney is aboard, at the height of his powers. Finney has not been very effective in most of his recent films, from the hysterics of ''The Dresser'' to Huston's own ''Annie.'' When faced with a meaningful task, though, he still has terrific resources to draw on. His portrayal of the Consul is overwhelmingly strong on both technical and emotional grounds, pungently tracing the character's weaknesses and failures while commanding our interest and eliciting our empathy. It's a bravura performance, well supported by Jacqueline Bisset as the Consul's long-suffering wife and Anthony Andrews as his puzzled half-brother.

In other respects, ''Under the Volcano'' is heavy-handed at times. The screen positively swarms with omens of the Consul's tragic fate. Certain minor characters are stereotyped, although most are sharply acted. Some of the details go beyond sordid to downright sleazy. The music, by Alex North, comes on too strong.

Yet it remains that ''Under the Volcano,'' a pet Huston project for around 30 years, has arrived powerfully and cogently onscreen. It's a glum affair, inevitably.

So was ''Volcano,'' the documentary that portrayed Lowry's own life, and which played in many American theaters a few years ago.

Still, the excellence of its execution is life-affirming; and its portrait of a time and place both threatening and seductive - as captured by veteran cinematographer Gabriel Figueroa - is unsurpassed in recent films.

This is not a happy work, but it demands a great deal of respect.