And for significant swathes of his quarter-century in the limelight, Kristofferson has actually been nowhere near it. Most of the work on which his reputation rests was concentrated in the 1970s, the singer-songwriter decade when the hunt was on for the new Dylan. (Before stardom, the closest Kristofferson got to Dylan was as a janitor in Nashville, when he cleaned out the studio ashtrays during the Blonde on Blonde sessions. Did he hang with Bob? "Nobody hung with Bob.") The story of his 1970s is pretty much told in the part he played in A Star Is Born, in which rock icon hits bottle and self-destruct button. In real life, he married not Barbra Streisand but Rita Coolidge, with whom, true to the rules of celebrity matrimony in Nashville, he cut a few albums as a preamble to messy divorce.
He made three films with Sam Peckinpah, one with Martin Scorsese. Scorsese obviously understood how cool a figure he was, because when Robert de Niro buys an LP to impress Cybill Shepherd in Taxi Driver it's a Kristofferson album. And she's so hip she already has it. "To this day I just think, what a sweet thing that was to do - the notion that I have Robert de Niro in a Martin Scorsese film holding up my album and quoting it and mentioning my name." For a while, then, Kristofferson got as close as anyone ever has to being pop star and film star in equal measure. "No," he demurs modestly. "Frank Sinatra did it."
At the summit of his career parabola he took star billing in Heaven's Gate, the flop of flops that pulled his Hollywood career under for years. In Final Cut, the page-turning post-mortem by producer Steven Bach, the author recalls the star showing up for the demoralising New York premiere with the wrong trousers. The whole year felt like that. "Everything fell apart in a year. My manager got Alzheimer's disease and my agent died and my family split apart and then just as I was trying to be a bachelor father, the film was blown out of the water."
Those clear blue Swedish eyes, set deep in a concave Celtic face, take in the copy of Final Cut awaiting his signature ("Thanks. Peace. Kris Kristofferson"). "I haven't had the stomach to read past a few pages of that. I got as far as the part where somebody was telling [director] Michael Cimino that they didn't like the choice of Isabelle Huppert because both Chris Walken and Kris Kristofferson are prettier than she is. He was trying to make a real piece of art, and he was fightin' the philistines the whole fuckin' way."
Professionally, there wasn't much solace in the early 1990s either, when he cut an album called Third World Warrior to get off his chest views on Iraq, Cuba, Nicaragua and other popular American holiday locations. "It was murder in my name," he says of US foreign policy, "with my taxes that was paying for it." Sympathisers scouring the radio for his songs twiddled the dial in vain. When Sinead O'Connor was booed offstage at Madison Square Gardens during the Dylan tribute in 1993, it was Kristofferson, ever the underdog's friend, who publicly roped a comforting arm round her. By then he didn't have a recording contract to call his own.
Then last year he made an album with Don Was, and a movie with John Sayles. Lone Star opens in the States this week, and finds Kristofferson playing "a racist sheriff in a Texas border town who is particularly murderous against blacks and chicanos. My wife said it wasn't a real stretch. I feel a great sense of gratitude," he adds, "that at my age whatever obstacles there were between the time of Heaven's Gate and now are not, that someone's willing to take a chance."
To push the album, he has just finished a month-long European tour with no nights off, and that raggedy, groaning voice is shot to bits ("How can you tell?" as Willie Nelson once quipped to him; when he started out, Kristofferson wasn't even allowed to sing on his own demos). But by the time he got to the Mean Fiddler last weekend he was still having a ball. The highlight came not with one of the many old, much-covered standards - "Help Me Make It through the Night", "Sunday Morning Coming Down", "For the Good Times" - but a new one called "The Promise", a growly dirge about love and learning and how at his time of life this father of eight is beyond improvement.
So what brought about the depoliticising of his songwriting? "My albums have been a reflection of whatever is going on in my life at the time. And, fortunately for me and the world, the life is better now. I have a real happy family life that I would never have predicted I could ever enjoy."
Five years ago, he moved his third family from Los Angeles - "like raising kids in a war-zone" - to Hawaii. Kristofferson's own youth was comfortable but peripatetic, his father being high up in the Air Force. A Rhodes scholarship brought him to Merton College, Oxford, where he got a degree in English, "which means you're qualified for absolutely nothing", and boxed for the university.
He spins a good boxing yarn, including one about his little-known association with Henry Cooper. "He worked in a place called the Thomas a Becket. They let me work out at the gym up there. I got to watch Henry spar with his brother. He was a good man. In fact, I ran into him once in a street in Soho when Paul Lincoln and I were coming out. Paul says, 'Goddamn, where's the camera right now?' "
Lincoln was Tommy Steele's manager, who had placed an ad in the Daily Mirror seeking musical talent. Kristofferson, who wrote his first song at the age of 11 - "an imitation country song" called "I Hate Your Ugly Face" - answered it. He'd already recorded a song or two in the States, so changed his name in London to Kris Carson. The results, produced by Tony Hatch, "were awful. I just wasn't up to it. I guess Paul figured with the PR possibilities of a Yank boxing at Oxford and that everything else in music at the time was bullshit he might as well do this one, too."
Back in the States, and after a stint in the army, Kristofferson "decided to start at the bottom and work my way up". A first marriage came and went while he worked as a helicopter pilot ferrying workers to the Gulf of Mexico oil-fields and commuted up to Nashville to sell songs. His employers didn't like that, or his drinking, and fired him. His first-born, meanwhile, was in hospital with a birth defect that needed $10,000 worth of treatment, and he faced jail for falling behind on child support.
This was the first of the trademark Kristofferson slumps that he seems to find oddly improving, even inspiring. "Me and Bobby McGee" improbably grew out of a parallel moment of despair in Fellini's La Strada, and the scene when Anthony Quinn realises that Giulietta Massina is dead. "He goes off and he gets drunk and he ends up on the beach howling at the stars and he was free but he was the most lonely son of a bitch in the world. So it showed the two sides of freedom. Freedom" - and this may as well be his career motto - "is just another word for nothing left to lose."
Out of curiosity, I ring up Henry Cooper and ask him if he recalls an American amateur sparring at the Thomas a Becket in the late 1950s, and whether he knew what became of him. Apologetically, Cooper dredges up a vague memory. "He was a charming guy, and he loved his boxing." Which is probably why he's never written about it: Kristofferson's songs are all about having a bad time, even thrilling on it. It's no fluke that whenever the conversation gets round to some cataclysm or other, it's usually punctuated by a rasping explosion of laughter. A Mexican boxer once told him that "a Mexican boxer will never give up". Has he carried that message out of the ring and into the rest of his absurdly colourful life?
"Absolutely. If you're trying to win, you can't really lose." And it seems to have worked. This may be a cliche, but if he whipped 10 candles off his birthday cake today, only then would one be able to say he was looking his age.