Whether it is or not, to film on location was an inspired decision which, with the unconventional but equally inspired casting of Humphrey Bogart and Katharine Hepburn, made The African Queen an exceptional movie. Bogart as an ignorant, gin-soaked riverboat captain; Hepburn as a dried-up spinster, a prim, forbidding English missionary? No, no, you think, only Hollywood could have come up with anything so preposterous. And yet it works. As we watch this unlikely couple fleeing from the German forces in the First World War, making their hazardous way down a barely navigable river, taking Bogart's boat, The African Queen, through perilous rapids while at first despising each other, then gradually, reluctantly, falling in love, disbelief virtually suspends itself. No effort is required because the splendour - and obvious discomfort - of the locations is clearly genuine and because these people, changing and growing, discovering within themselves and each other qualities of strength and resourcefulness that neither had suspected, are eminently credible.

There is romance here, danger, excitement and a leavening humour that springs naturally from the characters and the situations. Admittedly the climax, the daring assault on the German cruiser, is sentimental, glib and - let's be honest - highly implausible. But above all it is right. Nothing less than a triumphant ending would have been remotely acceptable.

Incidentally, Peter Viertel worked, uncredited, on the script. His book, one of the best novels about the movie industry, was filmed in 1990 with Clint Eastwood as the thinly disguised Huston. Both on page and on screen it makes an interesting companion piece to The African Queen itself.