Wayne had turned the film down, as had Steve McQueen, Robert Mitchum and various others. Frank Sinatra was set to star until, according to showbiz lore, tendonitis in his wrist prevented him from handling the Magnum's heavy recoil. "Probably just b******t," says Eastwood. But Ol' Blue Eyes' loss was Young Blue Eyes' gain. Eastwood brought director/collaborator Don Siegel to the project. And, courtesy of a much misquoted line - "You've got to ask yourself one question: do I feel lucky? Well do ya, punk?" - the picture turned Eastwood from cowboy star into everyman icon.
That same year, Eastwood directed his first film, Play Misty for Me. With Dirty Harry having established him as Warner Brothers' surest banker, he negotiated a quid pro quo: the studio would indulge his personal projects, such as Bronco Billy or Honkytonk Man, the kind of fare that would shape him as the director we know today, as long as he kept on cranking out the blockbusters, even if that meant working with an orangutan.
Sergio Leone, who directed Eastwood in his breakthrough role in the Man With No Name trilogy of spaghetti westerns, said he liked the actor because he had only two expressions: "one with the hat, one without it". These days it would be stretching it to suggest that Eastwood's range is quite that broad, his face seemingly fixed in a beatific beam, the sort of blissful countenance that once had him pegged in a scurrilous - and erroneous - piece of showbiz gossip as Stan Laurel's love child. The skin on his cheeks certainly seems tauter than one might expect of a man of his vintage. The contentment of his autumn years or the proverbial "bit of work"? Frankly, you can only wonder.
Nevertheless, he's imposingly tall (6ft 2in), sporty-lean, and could probably knock both 10 years off the 78 he has clocked up and seven bells out of anyone who messes with him, the result of relentless exercising, a strict diet and, probably, fatherhood late in life. In an arrangement at which even Ken Livingstone might raise eyebrows, Eastwood has had seven children with five different women, including an 11-year-old daughter with his current wife, Dina. It surely accounts for the emotional content of some of his recent films, not least Changeling, which had been in competition for the Palme d'Or and, like the lauded Mystic River, concerns child abduction.
There are actually echoes of Dirty Harry in Changeling, Eastwood says, and he's not making any concessions to liberals: "I get a kick out of it because the judge convicts the killer to two years in solitary confinement, and then to be hanged. In 1928 they said: 'You can spend two years thinking about it and then we're going to kill you.' Nowadays they're sitting there worrying about how putting a needle in is a cruel and unusual punishment, the same needle you would have if you had a blood test.
The politics are evidently always simmering with Eastwood. By the time Ronald Reagan was in the White House quoting Eastwood's "Go ahead, make my day" from Sudden Impact in a speech about tax cuts ("I must have heard it about 10,000 times," says Eastwood), he was shaping up to become the non-partisan mayor of the California town of Carmel, where he was sympathetic to environmental concerns and less sympathetic to big business.
Eastwood still likes to let his views be known, often forcefully. In 2005, he vowed he'd kill Michael Moore if the documentarian ever showed up at his house, the way he had doorstepped Charlton Heston in Bowling for Columbine. This March he was sacked from Governor Arnold Schwarzenegger's California state parks commission for objecting to the building of a toll road through a national forest. But though he has been associated in the public mind with Republican viewpoints, he's something of an individualist. "I don't pay attention to either side," he claims. "I mean, I've always been a libertarian. Leave everybody alone. Let everybody else do what they want. Just stay out of everybody else's hair. So I believe in that value of smaller government. Give politicians power and all of a sudden they'll misuse it on ya.
Has he declared for anybody in this electoral cycle? "You know, I haven't really," he says. "My wife used to be an anchorwoman in Arizona, so she knew John McCain and she liked him and I kinda liked him. In fact, we sort of supported him when he was running the first time against Bush eight years ago. But we haven't been active as yet. It's kind of a zoo out there right now. So I think I'll kinda let things percolate.
These days Eastwood doesn't really look back on his old films, though he mentions a viewing of The Outlaw Josey Wales, a film some regard as his masterpiece. He meant to watch for five minutes, but ended up sitting all the way through. "The films that I've done in recent years are the ones I remember the most," he says. "I guess I'm living in the present more than the past.
One thing he has made clear is that he will definitely not be making Dirty Harry 6, despite rumours to the contrary. "Some idiot came up with some theory," he says. The crime flick Gran Torino, which he is due to film at some point, is emphatically not part of the Dirty Harry cycle. "Not at my age," he stresses. "There are certain age limits on police officers. They'd have retired me out at 65.
But there's one film project on the cards that might interest Spike Lee. Eastwood's next project, The Human Factor, is about Nelson Mandela and how he used the country's victory in the 1995 Rugby World Cup as a means of fostering national unity. Will he be sticking with the historical record on that one? He laughs. "Yeah, I'm not going to make Nelson Mandela a white guy.
· The Dirty Harry Ultimate Collector's Edition box set is released on Monday