It’s exciting to see Shyamalan on such confident footing once more, all these years later.
The lady in the book is the mistress of a broken-down Haitian hotel owner who broods about politics and religion and, mostly, his mistress. This is because he hardly ever gets to see her. But in the movie he (Burton, that is) sees a lot more of her because, baby, when you're paying Liz Taylor's salary you really use her in your movie.
In the novel, the mistress is important mainly in the man's own mind. She represents his last chance at love, and it's pretty clear she's not going to make it much of a chance. She's married to the German ambassador (Peter Ustinov), has a son she loves and is also rather attracted to Major Brown (Alec Guinness), a dashing mercenary soldier and arms peddler. The hotel owner broods and, broods.
In the movie, he broods a lot but it's harder to understand why, since Liz pops up every 10 minutes or so and he can hardly find time most days to get his brooding properly brooded. The mistress' role has been expanded in the movie without exactly being deepened; most of the Taylor-Burton scenes consist of Taylor looking ravishing while Burton broods and whispers things to his tie knot. Who knows what he was whispering? Maybe it was dialog.