ON the night "A Streetcar Named Desire" opened on Broadway, Tennessee Williams sent his young leading man a rapturous telegram: "From the greasy Polack you will someday arrive at the gloomy Dane, for you have something that makes the theater a world of great possibilities."

Looking back now, you might describe that as, word for word, the most poignant couple of lines Williams ever wrote.

For one thing, "greasy Polack" reflects a pinched view of what Marlon Brando achieved. Stanley Kowalski is a brute, a vulgarian and a rapist, but Brando also gave him a canny intelligence and enough charm that the play's audiences joined him in laughing at Williams's heroine, Blanche DuBois, every night. Brando's looks also helped: thanks to the poetic face he carried atop his muscled body, his loutish Stanley could have passed for a slumming demigod.

In the end, of course, Brando never played Hamlet, nor did he exhaust the "great possibilities" that Williams and so many others detected.

Though he liberated generations of actors when he brought a fresh vulnerability to his early film roles - a majestic four-year run culminating in his 1954 portrayal of Terry Malloy, the anguished ex-boxer in "On the Waterfront" - Brando had barely reached his 30s before he entered his Elvis-in-the-jumpsuit phase.

He picked bad projects and gave indifferent performances, however speckled they might be with astonishing flashes. His weight ballooned, and he refused to learn his lines. Acting itself seemed ridiculous to him: "a bum's life," useful primarily as a way to pay his shrink's bills.

After more than a decade of this dud work, Brando made an astounding comeback, putting out "The Godfather" and "Last Tango in Paris" in 1972.

But the eccentric decay soon resumed. Nobody needed to see his lip-lock with Larry King, or his various family tragedies, or "The Island of Dr Moreau."

Why do the great ones so rarely have the capacity to handle their genius? Brando is not the first talent to invite the question - his contemporary Orson Welles preceded him on the path of brilliant promise, wobbly mature work and self-sabotaging obesity - but Stefan Kanfer helps establish him as the all-time Exhibit A.

His new biography, "Somebody: The Reckless Life and Remarkable Career of Marlon Brando," is not as exhaustive as Peter Manso's 1,100-page endeavor, or as concisely illuminating as Patricia Bosworth's more recent short book, or as engaging as Brando's own memoir. But it is the first serious biography to appear since Brando's death in 2004, and therefore the first account to corral in one place the whole story of what Kanfer aptly calls "a life of ludicrous excess, outlandish triumphs and appalling sorrows."

The combination didn't take long to emerge. Brando arrived in New York in 1943 at the age of 19, a military-school dropout from a busted Midwestern family.

His mother, a frustrated actress, drank; his father derided and brutalized him, reacting to his 4-F status in the war by sneering, "Is there anything else you could fail at?" But Brando found his way to the classes of the legendary Stella Adler, who soon declared that "this puppy thing will be America's finest actor."

His time at the New School is important and poorly understood. Though Brando's talent is frequently treated as a vindication of the Method, the memory-mining technique that Lee Strasberg sloppily adopted from the great Russian director Konstantin Stanislavsky, Kanfer points out that Brando despised Strasberg.

He learned much more from Adler, though she knew better than to take credit for his success. Stanislavsky's system is meant to help actors find the inspiration that geniuses feel with no system at all. Even as a novice, Brando made other actors look as if they were painting with rollers.

Radical as Brando's charismatic immediacy seemed to people on Broadway - and, beginning in 1950, to movie audiences - he wasn't the only one making such a breakthrough.

Around the same time, Jackson Pollock's art and Charlie Parker's jazz music reflected a similar mix of casualness and intensity, technique and spontaneity.

His personal life also grew chaotic enough to rival those of his fellow modernist giants, as he roamed nocturnal New York (particularly its minority neighborhoods), tended a pet raccoon named Russell and bedded everything that moved. But unlike Parker and Pollock - or his only acting rivals in those years, James Dean (who revered him) and Montgomery Clift - Brando didn't destroy himself, at least not all at once.

He let himself be humiliated by Truman Capote in a 1957 interview in The New Yorker; unwisely entrusted his money to his father, who lost it; and collected enough lovers and children (at least 11, by the end) to keep him in almost constant need of cash.

It is customary at this point to tut-tut about the ways that American culture forces its leading lights to work on execrable junk to remain solvent. But Kanfer, who has written biographies of Groucho Marx and Lucille Ball, notes that Brando made dumb choices, turning down premieres of "The Iceman Cometh" and "Present Laughter" on Broadway and "Lawrence of Arabia" and "Butch Cassidy and the Sundance Kid" on screen.

However much he abused them, Brando's gifts survived the rot of his later films. Watch, in "The Godfather," how he sniffs a rose, or strokes a cat, or slaps a co-star - all on-the-spot inventions of a fertile creative mind. To the end, he was "the same man with the same extraordinary aptitude for inhabiting a character, just older and heavier," Kanfer writes.

Though it provides a thorough account of Brando's life, Kanfer's book features little fresh insight, no matter how up-to-date its diagnoses - "oppositional defiant disorder," "narcissistic personality disorder" and the old standby "oral fixation" - may be.

In fact, it's precisely when Kanfer offers what should be the book's punch line that its limitations lie most plain: "If there was a 'Rosebud' in Brando's life," he writes in his concluding chapter, "it was the mental illness that had dogged him for decades, probably from early childhood." No kidding.

Kanfer's survey of Brando's post-humous legacy also feels a little wanting, reaching as it does to obscure Websites and pop lyrics to round out the picture.

But his legacy is, in a real sense, on every stage and screen in the world and thinking about it now leads to one name above all others: Heath Ledger.

As an introverted gay ranch hand in "Brokeback Mountain" and the quivering, maniacal Joker in "The Dark Knight," he touched the far extremes of a film actor's range, and made both look as natural as Brando in his prime. There's little point in wondering if he might have been the new Brando had he lived. As Elaine Stritch knew, there could be no such thing. Better simply to marvel that an actor so young found a way, as Brando did, to disappear into his art while remaining originally and brilliantly himself.