In Tintorettohs awesome "Crucifixion," a puppy waits patiently while his master digs a hole for one of the thieves' crosses. Above them, stared at by the multitude, Christ dies.

In "Madame Butterfly," as Cho-Cho-San's heart breaks, we hear far away and unrelatedly the sailors singing chanties in Nagasaki harbor. Whatever the artistic importance of the work, it is this mastery of byplay that often points up the difference between mere craftsman and artist. For the seemingly incidental in art is what helps make it so lifelike.

The best spy thriller writers have always understood this, knowing as they do the burdens of competition and plausibility they must overcome. In his new novel, "XPD" -- for Expedient Demise, as in instant death -- Len Deighton, for example, presents us with the grisly scene of two men on a bed, their heads and hands severed to prevent identification. A lesser writer would have hyped it by putting "Danse Macabre" or "O terra addio . . ." on the bedside radio. Deighton makes it Albeniz's guitar music.

Again, at a crucial meeting of British leaders, the prime minister's representative, when he learns that the world is in danger of falling apart, blurts out miserably: "It would mean the end of the Tory Party. That's what I can't bear thinking about."

How like the real-life words of one of our prominent congressmen when he was told by a reporter he had been discovered trying to bribe a reluctant ghetto boy into homosexual acts. The legislator's first thoughts were not of his family or even the possible felony charge. "My God," he exclaimed in anguish. "I'll never make it to the Senate!"

By definition, a spy thriller must thrill. We do not seek in Deighton or John Le Carre or Helen MacInnes the austere world of Dreiser or Joyce Carol Oates. We do not even want the real world of spying.

For there one finds dull CIA agents unable to spell the names of the art galleries or squares into which they gumshoe their quarries. (In a CIA report we obtained, the agent proudly tells of tracking his man into "Fargut" square at 17th and K.)

No, to meet the market, espionage thrillers are almost by nature improbable.

They amaze us with the deaths of kings, the heist of hydrogen bombs, with coups and genocides -- momentous chronicles in constant volcanic eruption.

In "SS-GB," Deighton's immediately previous novel, for instance, the Nazis have won World War II and occupy Britain. A single honest cop holds the fate of Hitler's empire in his hands. The premise of "XPD" is surrealistic. In the last days of World War II, a dirty dozen of American soldiers steal the Nazis' gold hoard and top secret documents from a mine. At the time of the novel's main action, 34 years later, we learn the documents disclose a Gordian Hitler-Churchill agreement which will destabilize the entire world.

For openers, one must point out that Deighton does not do this story to perfection. He annoys us by having one character tell another things that person already knows. By putting this explicative material in quotes, the page looks snappier. But a paraphrase would have shown more respect for the reader.

And surely, the American edition's 65,000-copy first printing could have been edited to remove such Anglicisms as "post" for "mail" when the action is in the United States and "Scotland Yard are looking . . ." when the speakers are Americans. (Does a finicky Washingtonian dare also question Deighton's designation of the Hay-Adams' restaurant as one of our "best"? Good, yes, but . . . And why is the District line called a "state line" when we can't even get the vote for Walter Fauntroy?)

These quibbles aside, we know from the first paragraph that we are comfortably in the hands of a thriller king. The terse scene-setting in Los Angeles, Geneva, Washington, London, Moscow, Denmark feel and smell of these locales.

The research on exotic guns, cars, poisons, trains, wall safes, foliage is shining and satisfying evidence of the hard work Deighton has done to make his background genuine and informative.

Above all, there are the little touches, the quick breaths that inspire this excellent book with its special life. Deighton so easily might have plunged the British spymaster into his most perilous case without a pause. Instead, the supersleuth stops a moment to consider whether to water his potted cactus. His KGB opposite, before deep-sixing a factotum, muses that he never really liked the man anyway because he wears elevator shoes.

Even on the book's final page, Deighton does not rush to denouement. The hero prepares dispassionately to murder the villain in a motel near Mount Vernon. He ponders whether, if he does the job well, he can get away with flying back to London on the Con corde rather than the prescribed tourist class.

Such grace notes are not unique, of course, to the spy novel. W. H. Auden points to the brillance of Brueghel whose Icarus falls from heaven as a farmer plows on, uncaring that a timeless myth is being born. Deighton, I am sure, would not rank himself among the Brueghels of art. Espionage writers, even the Eric Amblers, are somewhere between calligraphers and flower arrangers in the overall field of art. But there's a lot to be said for a man who can do flower arrangements around the mouth of an erupting volcano.