The project began life as a modest novel in 1935 by C. S. Forester, who would find his life’s work only two years later when he created the equally beloved Horatio Hornblower.

The plot of The African Queen is a straightforward up-the-river mission. After the First World War breaks out, and Rose’s missionary brother is killed by the Germans, she cajoles Allnut into rigging his boat as a torpedo to seek out and sink a German battleship guarding a vital lake.

Forester’s novel is an interesting mix of slightly stodgy adventure and surprisingly forward-thinking psychology, gingered up with dollops of well-travelled sailor lore. Allnut is a pale weakling with more than a taste for gin and no ability to withstand pain. He is well versed in the brothels of the world (“the enslaved prostitutes of the Port Said”, no less) and has a black mistress. Bogart’s roguish panache in the role is all the actor’s invention.

Similarly, Rose in the novel is more frumpy than Hepburn, and plumper too, which adds a different slant to their romance. She tidies him up, throws away his gin, and generally makes a man of him, but the Freudian implications of Allnut and Rose’s romance in print lie about as deep as the waiting crocodiles. More than once she clutches Allnut’s shrimpy frame to her capacious maternal breast to bring him some “peace and comfort”.

The 16-year journey from novel to screen, began with a 1938 attempt to co-star Bette Davis and David Niven. Nearly a decade later, the combination of Bette Davis and James Mason was tried but scuppered by her pregnancy. By 1949, the producer Sam Spiegel had hoodwinked Hepburn and Bogart, telling each one the other had already signed.

A new script was commissioned, with Huston using the film critic James Agee as a collaborator. Agee had ingratiated himself into Huston’s affections with laudatory reviews and glowing profiles, but he also liked to drink and converse on the masculine prerogative so Huston hired him.

The first hiccup was Agee’s elaborately literary approach (he was also a poet). “Oh Christ, Jim,” said Huston. “This is a screenplay. You’ve got to demonstrate everything. People on the screen are gods and goddesses. We know all about them. But we can’t touch them. They’re (already) symbols. You can’t have symbolism within symbolism.”

Agee learnt much working with Huston and praised his “good craftsmanship and taste”. But Agee keeled over with a heart attack and had to bow out, to his great embarrassment. Huston brought in the novelist Peter Viertel to refine the script, which lacked a strong ending.

Forester’s novel was not much help. In his US edition, he had the battleship sunk by another ship and Rose’s plans for marriage undermined by Allnut’s admission of an undivorced wife in South Africa. Forester rewrote a more upbeat ending for the British edition, but Agee’s draft of the script didn’t even include the near-hanging or wedding.

Huston’s final choice was between a downbeat ending, in which the couple were married then hanged, and the upbeat ending in which the German battleship hits the floating hull of the African Queen and sinks just before the hanging can be completed.

Huston opted for the lighter finale because he was “too bored to think of a good ending”. The result is a classic deus ex machina made palatable by the immortal line, “By the authority vested in me by Kaiser William II, I pronounce you husband and wife. Proceed with the execution.” But it is also made less forgiveable by the escape that closes Act II when sudden rain saves the couple from certain death after their boat gets stuck in the swamp of reeds.

The script’s biggest problem, though, was having Allnut and Rose consummate their relationship so early. It is the same in the novel. After they make it past the German port, their exhilaration turns into physical attraction. Thereafter the chalk and cheese friction takes a back seat as the effectively married pair solve external problems — bugs, leeches, swamps, a broken propeller etc. Their marriage at the end is only a confirmation of what we’ve seen, not a breakthrough of commitment.

It’s a shame as Allnut and Rose are a perfectly calibrated match of opposites who force each other to grow. She gives him a moral education, shaking off his scepticism to fight for a higher cause. He shows her excitement and sensual liberation, so she discovers life beyond her hymn book.

“It’s only human nature,” says Allnut when he gets drunk. “Nature, Mr Allnut,” she replies, “is what we are put in this world to rise above.” But he’s the one who rises and she’s the one who succumbs.

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