There is a lot of sexual imagery in the film - Bogart's entrance is announced by a shot of the mast of his boat against the sky accompanied by the toot of his horn. Having negotiated the first set of rapids, Hepburn famously says: "I never dreamed that mere physical experience could be so stimulating!" But the real sex in the film is the love affair the camera has with these two iconic, yet odd-looking, individuals. Hepburn transforms from old maid into something powerfully feminine; a precursor of Sigourney Weaver's Ripley, perhaps. Bogart is like we had not seen him up to this - vulnerable, not too clever and managing to be heroic in an everyday way e.g. stumbling into a war where he's not quite sure which way the various countries are aligned. He is often shot from below so that we see the prominence of his teeth; he is certainly no Clark Gable.
They were both wonderful actors. Certain tiny moments stand out. When, early on, Bogart returns to find her, with the village burned and her brother dead, she plays it as embarrassment - not logical but entirely believable - the shame we feel for losing things. Later, when Bogart is trying to save her he manages to pull the whole performance together in a few seconds acting behind her back while they are both facing the camera (I won't spoil the plot with more detail).
I'm not sure if the Blu-Ray edition released last year incorporates this version - sadly, I think not. However I believe it does contain a commentary from Jack Cardiff which should be worth a small investment when it hits the bargain shelves. So there, I knew I could write this without mentioning John Huston - Doh!