Here's how Phoenix describes his character: "I don't think he's a bad guy. He's obsessed with trying to create the ideal family. He's the product of a broken home and his tragedy is that the one person he looks to as a father figure betrays him.
"It's the dilemma of my generation. We're urged to pursue the American Dream, but we don't have the parental guidance to make life-altering decisions."
Phoenix has strong views on this. His parents, Arlyn Dunetz and John Bottom (who understandably changed his name), were hippy dreamers who spent the Seventies drifting through North, Central and South America. Joaquin Raphael Phoenix was born in Puerto Rico, the middle child of five (two boys, three girls), flanked on one side by the elder siblings, River Jude and Rainbow Joan Of Arc, and on the other by Liberty Butterfly and Summer Joy.
Conforming to nonconformity, Joaquin decided at three that he wanted a fancy moniker too, and called himself Leaf. The family liked to do everything together. "Whatever one of us did, the others just seemed to naturally gravitate towards. I'd get a skateboard, and my two younger sisters would buy skateboards too and come trailing behind me."
They returned to the US when Leaf was three. Money was tight, and at Christmas the children sang Beatles songs for nickels and dimes on the streets of Westwood, California. It was a shared dream to break into show business.
"We were always singing, playing music and painting. My brother started acting and we all pursued it, kind of simultaneously. So we looked for an agent who would take all five of us, because my parents didn't want to split us up."
They signed with Iris Burton, a tough-sounding cookie who later said, "Kids are pieces of meat. I've never had anything but filet mignon; I've never had hamburger." (Not a particularly apt metaphor in the case of the Phoenix clan, who were all vegans.)
Leaf was seven when he landed a small role on television, but he first attracted attention in the 1989 film comedy Parenthood. Then he dropped out. Good scripts were few and he preferred to travel with his father. Later he had a more compelling reason for staying out of the limelight.
Last week saw another, sadder anniversary: River's death from a drugs overdose at the age of 23. Joaquin (who had by then reverted to his birth name) was there that night - Halloween, 1993 - at Johnny Depp's Los Angeles club, the Viper Room.
His phone call to the emergency services was broadcast and published around the world. He was "shell-shocked" by the intrusion.
Asked now if it brought his family closer together, he mutters "We were always close," and with that, he makes it clear, the matter is closed.
One respects his reticence. After the tragedy, the media were full of threadbare homilies about the counter-cultural life style. Some painted the Phoenixes as pushy stage parents whose ambitions destroyed their son. Yet Joaquin talks of them with affection and unswerving loyalty.
"There's not much I would change about my life. What my parents instilled in me was family, love, relationships. It sounds simplistic, but that's how I was raised and these things really are important."
He's still with the same agent, who he describes as "very maternal". He still sees his family constantly. Rainbow (now called plain Rain) and Summer are still acting.
Joaquin returned to film acting in 1995, in the media satire To Die For. It was a poignant choice for a youngster trying to shake off the past. Not only was its subject the perils of celebrity, but its director, Gus Van Sant, had also cast River in his first important role, as a male prostitute in My Own Private Idaho, and later used Rain in Even Cowgirls Get the Blues. But Joaquin won praise for his touching and funny performance as a trailer-park teenager seduced by Nicole Kidman's brash weather girl.
I eye him across a roulette table in a (closed) Cannes casino. He's handsome, but in a more jagged, rough-edged way than River. Pale-green eyes contrast dramatically with his almost black hair, while a scar above his lip (a birth defect, he insists, not the legacy of a scrap) lends a vague menace.
Some interviewers describe him as "inarticulate" and "tense", and there's a sense of barely contained energy, or violence (a fight scene with Mark Wahlberg, his co-star in The Yards, left both battered). But he has a dry humour.
His brother once described him as "the family clown: very witty, very smart". Friends call him Joaq, as in "joke". But this year he has staked a strong claim to being taken seriously, with three contrasting movies.
One is the grittily contemporary The Yards. He was also the arrogant and neurotic emperor in Gladiator, whose success continues to surprise him. "I must be naive, because I thought, "I don't want to see this - it seems to be all about black leather and nickel-plated swords'."
He appears in Quills, the story of the Marquis de Sade. Towards the end of his life, de Sade (Geoffrey Rush, from Shine) was interned in the Charenton insane asylum. "I'm the priest and administrator, an optimist and idealist, with unorthodox methods," Joaquin says. "He uses artistic expression as a means of exorcising mental illness. But underneath all his seeming contentment are strong desires."
These fasten on de Sade's lusty yet virginal chambermaid, played by Kate Winslet. "If any two people should fall in love, it should be them, but this man is married to God. I'm a sex machine enclosed in this cassock. It's something that the Marquis sees in my character and tries to draw out. It's about how we repress our natural desires and how they sometimes manifest themselves in other ways that are more brutal."
Phoenix remains busy. He begins work soon on Buffalo Soldiers, based on Robert O'Connor's novel about American soldiers in Berlin just before the Wall came down. Will he be a winner in the roulette game of Hollywood?
The odds are good for an actor with certain talent, and one who has been maturing in the ascetic Nineties, not the excessive Eighties. Phoenix isn't laying any bets, however.
"I've no idea what they want, man," he groans. "I just try to get by."
The Daily Telegraph