When Sergio Leone went looking for an American actor, Clint Eastwood was last on his list. It was 1964, and Leone was about to embark on an unofficial remake of Akira Kurosawa's samurai picture Yojimbo. The ironic ronin Sanjuro Kuwabatake was now an ironic Western gunfighter named Joe. Leone's film was provisionally titled The Magnificent Stranger, and though it was a modestly budgeted spaghetti western, with locations in Madrid and Almería, the director had big ambitions for it. The producers wanted an American star, but Leone wanted more than that: he wanted an actor with the same toughness, wit and unpredictability as Toshiro Mifune, the star of Kurosawa's original film.

First choice for the producers was an expat American named Richard Harrison. They liked him because he lived in Rome, and they wouldn't have to fly him over. But his fee was a little high. For Leone, Harrison wasn't even on the list. His first choice was Henry Fonda, and he sent the script to Fonda's agent in Los Angeles, offering him the part of Joe.

Fonda's agent passed without showing his client the script. Leone approached three more excellent choices: Lee Marvin and Charles Bronson, both of whom rejected it, and James Coburn, who was interested but not for the offered fee of $15,000. A trawl for less-great actors began: Cameron Mitchell, Tony Kendall, Frank Wolff, Vassili Karis (who he?) and finally Richard Harrison. All turned it down. Harrison had already starred in a couple of spaghetti westerns, and thought there was no real future in them. The real action, he believed, was going to be gladiator films. But, seeing that Leone and co were in a spot, Harrison recommended another actor who might be interested.

Clint Eastwood was best known, at this point, as a television actor. He played a likeable cowpoke, "Rowdy" Yates, in a TV western series called Rawhide. His film experience was fairly minimal, though he had played a sidekick to Francis the Talking Mule. Eastwood was keen to get more film work; when Leone offered him the lead in the movie that would eventually be titled Per un pugno di dollari - A Fistful of Dollars - he accepted the fifteen grand.

When Eastwood arrived in Rome, Leone avoided him, pretending he was ill and sending his assistant, Mario Caiano, to meet the actor. A similar anxiety governed their on-set relations. According to Eastwood, the only word of English that Leone could say was "goodbye". So Eastwood and his stunt double, Bill Thompkins, communicated with their director through sign language and his bilingual stunt co-ordinator, Benito Stefanelli. In any case, Eastwood knew what Leone wanted, as Leone would famously mime all the action for his cast: how to walk, how to draw your pistol, how to hit someone.

And Eastwood learned something else, on the dusty old Zorro set outside Madrid - a lesson that would serve him brilliantly as an actor, and as the star he would soon become. The script's first draft was a huge document, heavy on the portentous dialogue. Eastwood knew he couldn't say the stuff in any case; but he also knew, instinctively, that he shouldn't. Resisting the usual actor's temptation to expand his role and to increase his share of the dialogue, Eastwood took a red pencil to the script. Something like 90 per cent of his scripted dialogue was eliminated. What remained counted: "Now, if you'll just apologise to my mule . . . like I know you're gonna . . ."

When A Fistful of Dollars opened in Italy, it enjoyed huge commercial success. It is also a fine film: beautifully lit and shot, with a marvellous score (by Ennio Morricone, disguised as "Dan Savio") and two strong antagonists: Clint Eastwood and Gian Maria Volontè. Volontè was an Italian actor of the all-the-stops-out school who had been blacklisted for his communism. He gives a tremendous performance, but it is Eastwood whom we root for: he remains a credible hero in the face of all odds, thanks in large measure to his taciturnity.

Eastwood shared his secret with Lee Van Cleef on the set of the official sequel, For a Few Dollars More (1965). Faced with page after page of overwritten dialogue, the two men pared it away - down to a few sentences, a couple of words, a look. The bad guy once again was Volontè, better than ever, playing a psychotic, pot-addicted killer called El Indio. And the Eastwood-Van Cleef team was a strong one: this was one of those rare occasions where the sequel is better than the original film.

For a Few Dollars More sold more tickets than any other film in Italian history. Another sequel, funded by an American studio, was inevitable. By now, Eastwood had grown a little tired of Leone's magic. The money kept getting better, and he owned a piece of the picture. But the shoots for The Good, the Bad and the Ugly (1966) were longer, and - at times - dangerous, as when Leone wanted Eastwood and Eli Wallach to crouch near a bridge that was to be blown up.

Originally Leone had planned to have Volontè appear in The Good, the Bad and the Ugly, but he fell heavily for Wallach's charms and cast him instead. Wallach's character, Tuco, dominates the film to such an extent that both Eastwood and Van Cleef tend to become his foils. Eastwood wasn't interested in this development. It was the last time he and Leone worked together.

In later years, Eastwood claimed that all had been chaos on Leone's sets, and that he, Eastwood, had invented his character and dressed himself, bringing his own gun belts and his signature poncho from Western Costume in Los Angeles. But the cultural historian Christopher Frayling has pointed out that the poncho, or serape, was included in Carlo Simi's design sketches for A Fistful of Dollars months before Eastwood and his trunks landed in Rome.

The falling-out between the actor and the director was perhaps inevitable. Eastwood owed Leone everything. He's good in A Fistful of Dollars - very good - but Lee Marvin would have been excellent, and Charles Bronson or James Coburn equally fine, in the same role. Eastwood was lucky to be cast.

After parting with Leone, he made an imitation spaghetti western, Hang 'Em High (1968), and looked set for a series of similar. His luck held, however, and he fell in with another great director, Don Siegel, who cast him in Dirty Harry and The Beguiled (both 1971). Eastwood's work with Siegel, as a right-wing, vengeful cop and a conniving Civil War deserter respectively, is the best I have seen him do. Since parting company with Siegel, Eastwood has acted in numerous films, none of them very memorable, usually portraying vengeful cowboys or the oldest officer in the San Francisco Police Department. Like Ronald Reagan, he has co-starred with apes. And, like many restless actors, Eastwood has decided he is also a director, though he has yet to develop an individual or interesting style. To me, all his directorial chops seem borrowed either from Leone or from his other mentor, Siegel.

Even Eastwood's directorial fixation on "mercy killing" seems to stem from his days with Leone. One of the ways Leone's westerns offended the Hays Code (Hollywood's self-imposed censorship guidelines) was the presence of such killings: in each film, Eastwood's character shoots a wounded man to finish him off. As a director, Eastwood got into a fight with the Pentagon over his first war movie, Heartbreak Ridge (1986). Detailing the glorious victory of the US military over a handful of Cuban construction workers in Grenada, Heartbreak Ridge had the full support of the Pentagon: free troops, free tanks, free helicopters, whatever Eastwood wanted. But he had to submit the script for official approval, and - you guessed it - the Pentagon had a problem right there on page 61: the mercy killing. So intent was Eastwood on including his trademark, even though it was fictitious, that he gave up the free stuff and made the studio pay for the tanks and planes and troops instead.

Now this might seem stand-up stuff, the brave director facing off an intrusive Pentagon's desire to suppress the truth. But it wasn't true; it was just important to Eastwood, for some perverse reason - just as it was important for him, late in his career, to direct a film espousing the euthanasia of a beautiful, crippled woman by an old man: Million-Dollar Baby (2004).

Try to figure out that psyche if you wish. I prefer to pull out the ol' videotape of For a Few Dollars More and watch that one again. Van Cleef looks of a certain age - he was 40 when he made the film; Leone thought he was 50.

But Eastwood, Volontè, Klaus Kinski and that great cast of bad guys . . . When I first saw those faces, I was a teenager. They looked like gods; they seemed ageless. Now they look like young fellas, in their thirties. What happened? Where are my gods?

A season of films directed by Clint Eastwood runs until 31 August at the BFI Southbank, London SE1. www.bfi.org.uk

Alex Cox's "X-Films: True Confessions of a Radical Film-maker" (I B Tauris, £18.99) is out now