Jonathan Nolan, the film's co-writer, and brother of director Christopher Nolan, recalled the outcry earlier this week in a round-table discussion convened by The Hollywood Reporter. "Everyone was coming to Chris and saying, 'We don't see it'," he recalled. "And the fan community - we were pilloried for it." Robin Williams, Mark Hamill and Johnny Depp were among the names circulated as the kind of actors Warner Bros should be casting - though now, it is impossible to picture anyone but Ledger in that moth-eaten purple overcoat and sweat-streaked pan-stick.

"For me, casting, beyond writing, is the most dangerous moment in every project," Nolan added. And the case of The Dark Knight shows how the soul of even the beefiest blockbuster depends on the alchemy of matching actor and part. It also came to light this week that Ledger had been far from the first choice for Brokeback Mountain: in an interview with IndieWire, the director Gus Van Sant revealed that Leonardo DiCaprio, Brad Pitt, Matt Damon and Ryan Phillippe had all passed on the project.

For critics, titbits like this inevitably prompt daydreams of the films that might have been, now screening in a parallel universe near you.

DiCaprio in Brokeback Mountain is a particularly intriguing case: when that film was being cast, the actor was in his late 20s, and still extraordinarily cautious about alienating the tranche of his fan base who had fallen for the cherubic heart-throb they saw in Romeo + Juliet. Homosexuals weren't part of the career plan - and, as for porn stars or serial killers, forget it. DiCaprio had first refusal on the leads in Boogie Nights and American Psycho, but turned them down to make the significantly more teen-girl-friendly Titanic and The Beach. He later said that sidestepping the former - Paul Thomas Anderson's exhilarating ensemble piece set in the disco-era adult movie business - remained his "biggest regret".

Then, last year, Mark Wahlberg, who did get the Boogie Nights part, announced, at a conference for young Catholics, that he regards it as "up there at the top of the list" of his "poor choices". That is quite something from the star of Ted.

These twists of moviemaking fate usually have far less to do with ideology than instinct. Watching Mark Kermode's excellent new series Secrets of Cinema on BBC Four this week, I learned that Richard Gere was chosen for Pretty Woman - a role he might have been grown in a lab to play - only after the studio decided against Al Pacino and Burt Reynolds. In fact, as originally written, Pretty Woman was a dark parable that would have suited particularly Pacino and his audition partner, Michelle Pfeiffer. But when Disney saw the chemistry pouring off Gere and Julia Roberts in their audition tape, the whole thing was reupholstered to fit.

A strong audition changed the course of Harrison Ford's career, too: impressively, it wasn't even his. When George Lucas was piecing together Star Wars in 1975, he imagined Kurt Russell as his swashbuckling smuggler, Han Solo. But Ford, who had gone back to carpentry after a small role in Lucas's American Graffiti, happened to be building a door in the office where the director was auditioning prospective Leias and Lukes. He asked Ford to come in and read Han's lines, and everything clicked.

In that parallel universe multiplex, Russell was probably a serviceable Solo. But Star Wars is what it is thanks, in no small part, to Ford - as are the Indiana Jones films, although that role almost went to Tom Selleck. It would be fascinating to drop down a wormhole and watch the version of Alien in which Meryl Streep played Ellen Ripley, or the Forrest Gump in which John Travolta sat on that park bench, the Jerry Maguire with Tom Hanks instead of Cruise, or the Dr No with Cary Grant as James Bond. Each of these films almost came to pass, until fate intervened. Or, as Karen Allen's Marion once almost said to Selleck's Indiana: something made it inevitable.

The Sunday Telegraph (London)