BBC Radio did a series of Christie mysteries a while back. Sad Cypress is one that I happened to come across. It's very well done and during the course of the drama, a woman sings "Sad Cypress"—kind of a 1920s jazz rendition--and it's fabulous. So I spent the whole movie waiting for someone to sing that song. But nobody did.
It was a curiously lifeless movie. The casting was fine, the script was fine, the acting was fine, the mystery was, well, odd. The theme, in keeping with the first two, was about the wretchedness of love. But the story never really came to life. I still recommend it, only to rent, not to buy.
I consider The Hollow to be one of the most complex of Christie's mysteries, not in terms of plot, but in terms of character. Christie is often belittled for creating one-dimensional characters; I think such criticism misses the genius of Christie's writing and what makes her so popular. Christie was a very intelligent woman who, nevertheless, opted for commonsense over intellectualism. The characters that she created were constructed out of her views on human nature: down to earth, sensible, commonsense views. This is what makes them recognizable. It isn't that she used a stereotype of say, a vicar; it is that she captured, in a few minor brushstrokes, a vicar-like type. You may not recognize him as an individual, but you nevertheless recognize him.
In The Hollow, Christie brought some of her most interesting "types" together, and she gave them reasonable motivations and backgrounds. She isn't Freudian. Reading Christie is like having a good gossip with your girlfriend. She may not be giving you DSM-like explanations but her explanations are ones that on a human level, you recognize. Yes, yes, people do behave like that.
My disappointment with the movie is that it really could have been a two parter: 3 hours. This is not true of most Christie novels to movies. Christie is one of the few novelists where everything can easily be condensed to an hour and a half: there just isn't that much material there. The Hollow is an exception. The Edward-Mig story was almost completely left out. John Cristow's research (which is important) and his patients were referred to only briefly. The casting was spot on, the script was good (the dialog between Poirot and Henrietta is excellent), but it suffered from the time constraint.
But again, I recommend it. There can never be too much Christie.