Not only is it one of the most 'psychological' of Christie's novels, the cast in this particular production was excellent. There's Donald Sinden - doing his most 'brllaa, brllaa' wobbly-jowled voice as Colonel Race.
|'brllaa, brllaa, I'm Donald Sinden, brllaa'|
And Stephanie Cole made a pleasant change as Ariadne Oliver, usually she's played as a ditsy, scatty eccentric (see the latest ITV Poirot series, and the Peter Ustinov-helmed 'Dead Man's Folly' from the eighties). But in this production she was realistic, honest and relatively normal - which makes the story more believable overall.
As for the psychology, well... Poirot asks each suspect what the others were like at playing bridge, and not only does their opinion show him what they really think about the other players but also what they themselves are like. What they notice in the surrounding room is what is important to them (historic artifacts or expensive jewellery). In the absence of any physical evidence, it's up to Poirot to figure out how and why the crime was committed, which he does with characteristic élan.