Every element of the film is finely balanced - heroes and villains, cynicism and idealism, tragedy and comedy. Everything is exactly in its place and precisely serves its purpose. Yet it could all have been so horribly different. It in no way detracts from director Michael Curtiz's judgement in making the calls he did that the film came together so perfectly largely by chance.

Although it is widely believed Ronald Reagan was seriously considered for the role of Rick Blaine, he probably wasn't. George Raft, however, certainly was, and Hedy Lamarr and Ann Sheridan were both on the shortlist for Ilsa.

Although the screenplay was based on an unproduced stage play, Everybody Comes to Rick's, the script was incomplete when the movie started shooting, and was subject to constant revision by a team of writers, each with a different idea of the right tone.

The fog that establishes the atmosphere for the climactic airport sequence was there not for that reason but to conceal an unconvincing model of a plane.

As Time Goes By would have been replaced by another song had Bergman not cut her hair, making it impossible to reshoot the sequences in which Sam plays it.

Even the famous final line, 'Louis, I think this is the beginning of a beautiful friendship', was an afterthought, contributed by producer Hal Wallis, who called Bogart back to overdub it.

Happy accidents and spur of the moment improvisation certainly played their part, but so did fine acting even though the on-screen chemistry between Bogart and Bergman did not reflect an attraction in real life. Neither Bergman, Bogart nor Paul Henreid, who played Ilsa's heroic but stiff husband Victor Laszlo, particularly liked each other, and as Bergman later said of Bogart: 'I kissed him, but I didn't know him.'

There is much more to Casablanca than the three stars, however. There is the dry humour of Claude Rains' corrupt chief of police, vintage sleazy performances from Bogart's co-stars in the previous year's The Maltese Falcon, Sydney Greenstreet and Peter Lorre, and Dooley Wilson as Sam.

There is a persistent myth that Bergman did not know which of the two male leads her character would leave Casablanca with but, despite all the script revisions, that question had been settled early in development. The moral climate of the times and the film's brief to boost a sense of patriotic duty in a time of war determined the outcome but, for Bergman at least, life did not imitate art.

Eight years after filming Casablanca, she fell in love with Italian director Roberto Rosselini on the set of Stromboli, and became pregnant. America was outraged and took years to forgive her - partly perhaps for failing to do what Ilsa did.

Bergman, for her part, resented the association with the role whose aura she seemed unable to escape, but eventually came to understand its magic for audiences. 'I feel about Casablanca that it has a life of its own,' she said later. 'There is something mystical about it. It seems to have filled a need, a need that was there before the film.'