Try to imagine The Wizard of Ozwith Shirley Temple, Casablancawith Ronald Reagan or Titanicwith Matthew McConaughey. Joe Griffinon the also-rans of movies

JUST as theatre can be beautiful because every performance is unique, one of the appealing aspects of cinema is its permanence. Harrison Ford is Indiana Jones. Casablanca is unimaginable without Bogart and Bergman.

But what if first choice Tom Selleck had played Indy? Would Ford be guesting on Friends? Would Sean Connery be an icon if David Niven hadn't asked for too much money to play James Bond? One imagines that, even if Tom Cruise hadn't made Top Gun, his ambition and career choices would probably have made him famous regardless, but nobody really knows.

There are other considerations. Would Heath Ledger have been so fatally exhausted had Adrian Brody beat him to role of The Joker in The Dark Knight? Would Brad Pitt have left Jennifer Aniston had he been acting opposite producer's choice Nicole Kidman in Mr and Mrs Smith? Would Paul Hogan still be a star had he not turned down Ghost?

Often the fate of the very film's existence rests in the actor's decision. Paul Thomas Anderson has admitted that had Daniel Day Lewis not been involved in There Will Be Blood (opening this week), the film may not have secured financing.

It's not enough for an actor to be talented and loved by the public. They must be able to predict what films will hit or miss, based on a pitch, a meeting, and a script. It can make the difference between having the career of Matt Damon or Ben Affleck.

Arnold Schwarzenegger, once the biggest star in the world, boasted that he had a "good nose". He knew, for example, to play the monotone villain instead of the hero in The Terminator, which led to a decade of hits.

Nobody can blame Colin Farrell for wanting to work with Oliver Stone and Michael Mann. It was just bad luck that Alexander and Miami Vice turned out to be so disappointing. Similarly, time will tell whether Ryan Gosling (starring in this week's Lars and the Real Girl) made the right choice by dropping out of Peter Jackson's The Lovely Bones.

Often the scraps from the table of one actor can be the career-defining role of another.