The trailer of Bennett Miller’s Capote might give one the idea that it’s just Philip Seymour Hoffman doing a ferocious impersonation of the writer – the piping voice, the name-dropping, the way his upper lip twitches à la Monroe and his fat tongue lolls in the corner of his mouth when he laughs. Even if that had been the extent of it, I would have rushed to see it – not because I cared that much about Truman Capote, but because Hoffman is always so electrifying to watch.

From its opening shots, though, Capote announces itself as something serious and profound. Soft, widely spaced piano chords accompany the film’s economical set-up. A field of wheat rustles in sepia light. A teenage girl in an overcoat knocks, and knocks again, at the door of a white timber farmhouse. She enters the house, mounts the stairs past the framed photos of its occupants, and introduces us, in one speechless reaction shot at a bedroom door, to the crux of the story that will furnish the material for Capote’s great “non-fiction novel”, In Cold Blood: the house is full of corpses. Four members of this Kansas farming family have been tied up and slaughtered point-blank with a shotgun.

In New York, Truman Capote, the already famous novelist and party-going show pony, sits alone in his apartment reading the newspaper. His face and body are quite still as he absorbs the bare facts of the Kansas report; his hand, as he clips out the item with an old pair of scissors, is very slow. Crunch, crunch go the heavy, dark blades, deliberate and merciless. Can it be – oh, can it be – that this whole thing is actually going to be about writing?

Writers in movies are usually embarrassing. Mediocre actors pace wildly through oceans of screwed-up paper, or hurl portable typewriters through windows. But we’re looking at the great Philip Seymour Hoffman, who understands that the state Janet Malcolm calls “the familiar stirrings of reportorial desire” is invisible because it’s so perfectly inward. Later, in Kansas, we see Capote enter an empty room in the sheriff’s residence. Just inside the door he stops and clasps his hands. Again, he remains motionless; but we sense his pores opening, his nerves aquiver, as if his whole being were sucking in detail. As Yeats put it, Like a long-legged fly upon the stream / His mind moves upon silence.

A fanciful creature like Truman Capote, with his ankle-length camelhair coat and lofty New Yorker manner, is not going to pass unnoticed in rural Kansas in 1959. He has the sense to take with him, as “research assistant and personal bodyguard”, his childhood friend from Alabama, Nelle Harper Lee, whose huge success with her novel To Kill a Mockingbird is still safely in the future. The handsome Catherine Keener plays Lee as a reserved, sensible character, in knitted cardigans and no make-up. As straight woman to Capote’s flashy manipulator, she gets him past the wariness of the provincial people and into their confidence. She gives off a steady glow of humorous affection and decency, against which Capote’s increasingly ruthless ambition flares all the more lurid.

You would hardly think that an egomaniac like Capote could be a natural interviewer. But his charm is lethal. He knows instinctively how to direct his narcissism, how to use it: when to let a silence go on and on, and when to break it with a painful revelation from his own childhood that will loosen a tongue in empathy.

There are times, especially at the dinner table of Detective Alvin Dewey (the wonderful Chris Cooper, purple-lipped and baggy-eyed in a narrow-brimmed felt hat, his lowered brow bulging with tension and fiercely suppressed rage), when Hoffman’s performance is terrifying. The risks he takes are almost obscene. The camera dwells on his face as he pushes deeper and deeper into outrageously intimate territory. Martini in hand, he teeters on the very lip of an abyss.

Two men are arrested in Las Vegas and brought back to Kansas to be charged with the crime. Capote and Lee join the townspeople on the cold steps of the courthouse to witness the perp walk. One of the prisoners, Perry Smith, a good-looking young man with dark hair and strangely withered legs, glances up and catches Capote’s eye. Something clicks between them. In that second, Capote is lost.

Smith and Dick Hickock confess. The movie does not concern itself with the trial. It cuts from a shot of the empanelled jury straight to the guilty verdict and the death sentences, and then to Capote’s fatal, self-interested promise to get Smith “a proper lawyer”.

Anyone who has read Janet Malcolm’s scathing book The Journalist and the Murderer will know that Capote, in making this rash offer, has stepped over the line into the territory where all he can do, in the end, is betray. He and Perry Smith, drawn to each other by contradictory needs, are soon locked into the mysterious dance of writer and subject, condemned to what Malcolm calls “the falseness that is built into (that) relationship, and about which nothing can be done.”

The condemned men want a retrial on grounds of insanity; Capote doesn’t disabuse them of their fantasy that his book might support their case. What he really wants, of course, is a detailed, first-person account of what happened on the night of the murders, and then a neat pair of executions so he can be quit of the story and the New Yorker can publish it. He doesn’t waste his time on Hickock, a thuggish, big-noting smart-arse; but the relationship that grows between Capote and Perry Smith, a psychopath with artistic fantasies and a large vocabulary, is disturbingly erotic, in the most complex sense. It makes our flesh creep to watch Capote manipulate Smith’s hopes. “He trusts me,” says Capote to Harper Lee. “He wants so much to be taken seriously – held in esteem.” “And do you?” asks Lee. There’s a long pause; then Capote whispers, “He’s a goldmine.”

The film’s achievement is to hold the repellent side of Capote’s personality in delicate balance with his seriousness as an artist. In Cold Blood took him four years to write. It appeared when he was barely forty. Rereading it, one finds a splendid work, disciplined, graceful, severe; his best by far. But after its enormous success he suffered an appalling twenty-year moral collapse. He never finished another book. What this movie brilliantly shows is the relentless labour and the ugly moral contortions he put himself through to create that master-work. Capote, with its mature authority of vision, its faultless performances and its forensic examination of character, is the finest film to come out of America in a long, long time.