What if... Jeff Bridges had got the part in Taxi Driver, if Ronald Reagan had smouldered in Casablanca, and Indiana Jones had worn a Magnum 'tache? Well, that was the original idea.

As great screen moments go, Frank Sinatra in the back of the taxi with Rod Steiger doing the "I coulda been a contender" speech in On the Waterfront vies with Jeff Bridges looking into a mirror repeating "You talkin' to me?" in Taxi Driver.

Frank Sinatra not Marlon Brando as Terry Molloy? Jeff Bridges not Robert De Niro as Travis Bickle? Here is an alternative film world where movie stars are relegated from the premier league because the parts that first made them screen idols are taken by the actors who originally turned them down.

Sharon Stone would still be stuck in B-movie hell instead of co-starring in Scorsese's new film Casino if Michelle Pfeiffer hadn't turned down Basic Instinct. And Jodie Foster wouldn't have her superstar status or Demi Moore her $12.5m asking price if Pfeiffer hadn't also passed on Silence of the Lambs and Ghost.

It's an old story. When Ronald Reagan turned down the part of Rick in Casablanca, he didn't realise that ex-Presidents are a dime a dozen, but you can count the number of genuine movie icons - Humphrey Bogart - on one hand.

Jack Nicholson was about to swap acting for scriptwriting and producing when he was offered the cameo in Easy Rider that Rip Torn had turned down. And Nicholson only became a major star because James Caan rejected the lead role in One Flew over the Cuckoo's Nest.

Harrison Ford might still be a carpenter if Jeff Bridges (again) and Tom Selleck hadn't turned down the part of Indiana Jones in Raiders of the Lost Ark. Robert Redford declined the part of the disaffected young man seduced by an older woman in The Graduate, allowing Dustin Hoffman (31 and ageing) a shot at stardom. In turn, Warren Beatty passed on The Great Gatsby and The Way We Were. Both films helped establish Redford as Hollywood's golden boy.

Of course, all these actors have "star quality" - the charisma and presence that made their success inevitable, didn't they? "There is a notion that 'star quality' will always show through," says Gene Hackman, who shared a flat in New York with Hoffman and Al Pacino in the Sixties when all three were scuffling for work. "But if you don't get the break, you can be the best actor in the business and it won't mean a thing. I'd done Bonnie and Clyde and some other films before French Connection established me [Hackman won an Oscar for his portrayal of Popeye Doyle]. I take no special credit for my performance. It was a terrific part, I was not the first choice for it - and anyone could have done it as well."

John Wayne's career shows that so-called star quality is less important for an actor's success than the right film at the right time. He made dozens of B-westerns before the public responded to him in Stagecoach. And after attracting little notice in several earlier outings, Julia Roberts was catapulted to stardom in Pretty Woman.

Getting the right film usually depends on someone else turning a part down first. Dudley Moore had his brief place in the Hollywood sun thanks to George Segal, who turned down 10. Jon Voight became a star with Midnight Cowboy after his part of a male prostitute was turned down by Elvis Presley.

Often an actor has put his or her stamp so firmly on a breakthrough role that it's hard to imagine anyone else being considered for it. Peter O'Toole is Lawrence of Arabia - but only because first choice, Albert Finney, backed out after four days of filming, and second choice, Marlon Brando, also said no.

Who else but Robert Redford could have played the Sundance Kid to Paul Newman's Butch Cassidy? Well, it should have been Steve McQueen but he decided against it. The Godfather made Al Pacino a star and Pacino as Michael Corleone made the Godfather trilogy great. But the film's producers wanted almost anybody else for the part until Brando and the director, Francis Ford Coppola, insisted.

Established actors have made characters their own. Who else but Bogart and Katherine Hepburn could star together in The African Queen? Well, it was intended as a vehicle for John Mills and Bette Davis. The Road to... movies fit Bob Hope and Bing Crosby like a glove, although the glove had been intended for Fred McMurray and George Burns.

Actors at the top of the Hollywood food chain always get the pick of the choicest parts. As these A-list actors can only commit to a fraction of the projects offered to them, it means that virtually all the leading parts in Hollywood films have been turned down before they find a taker. At any given time one or two actors are particularly "hot". A couple of years back there were eight Robin Hood projects doing the rounds, and Kevin Costner had been offered the lead in all of them. These days Brad Pitt gets first choice at everything.

Most actors aspire to the A-list if only to get first crack at the good parts. But choosing the film that will give them there break is not a scientific process. As the scriptwriter William Goldman points out: "Actors don't know if a film is going to be a hit. They look at the budget, the director, the script, but they can't be sure. So then they look at how big their part is, at how it will affect their image. And if they have any doubts, if they are a reasonably successful actor, they'll say no sooner than yes."

But by saying no, they can miss out on some major successes. Jeff Bridges, who admits to turning down another "enormous" picture apart from Raiders and Taxi Driver, says: "It's not worth talking about what you've passed on because we've all done it. It's just part of the business. And there is no way of knowing if the film you turn down would have been as big if you had been in it. The other guy maybe brought something to the part that was just right; and you would have played it differently and it might not have taken."

Tell that to his brother Beau who turned down the Seventies hit Love Story, or John Travolta before Pulp Fiction salvaged his career. He turned down Days of Heaven, American Gigolo and An Officer and a Gentleman. Richard Gere became a star on the strength of them.

Usually it doesn't matter who turns down what. It's best not to wonder how Bridges, usually a fine actor, might have played Bickle, given his pop-eyed, slack-jawed performance as a wacko in The Vanishing. But movie buffs might wonder how Cary Grant would have compared to Rex Harrison as Professor Higgins in My Fair Lady. Or how De Niro would have played Tom Hanks's role of the little boy in the body of a man in Big.

John Crawford not Deborah Kerr should have been clinching on the beach with Burt Lancaster in From Here to Eternity, but Crawford didn't like the dresses she was shown at the costume fittings.

John Houston's film of Kipling's The Man Who Would Be King is probably unique in that it was recast over a period of 30 years. He first wanted to make the film in the early Fifties when his two adventurers were to be played by Gable and Bogart. In the Sixties it was to be Richard Burton and Peter O'Toole. Eventually, in the Seventies, Michael Caine and Sean Connery played them to perfection.

Woody Allen, however, has the last word on recasting. He shot his 1987 film September twice, with different actors in two major parts. The first time round Maureen O'Sullivan played Mia Farrow's mother (as she is in real life) and Christopher Walken played the man who loved her. Dissatisfied with the results, Allen did the whole thing again with Elaine Stritch and Sam Waterston in the parts. It didn't help, the film was still a dud.