The performances are all satisfactory without any being exceptional. Joan Hickson has a presence about equal in impact to a tuna fish on white bread but other reviewers have compared her acting favorably with that of Helen Hayes in an earlier version of the story, and I can believe it.
Donald Pleasance, whose bald head has become freckled with age, is great as the wheelchair-bound curmudgeon, and Barbara Barnes as his major domo is properly pretty. As Lucky Dyson, Sue Loyd is presented as repellent but Sophie Ward as the endangered wife is flawless -- beautiful, slender, frangible, edible. Her acting is okay too. But it's really a little unfair that someone should be as attractive as Ward, way out there on the end of the Gaussian distribution, while so many of the rest of us fall short by a silly millimeter here, a tangent there, a conical section somewhere else. What is it that makes one person gorgeous and another not? Inquiring minds want to know. And, curse her, she moves with elegance and grace too. Has she no modesty?
As for the plot, well, I read the novel years ago and found it confusing and gossipy and the film doesn't change things much. I actually didn't understand much of it. Partly because of the intricacy of the story itself, partly because of some odd accents used by the principals, and partly because those damned chirping crickets didn't sound like crickets at all, but rather the squeakings of a rusty old porch swing -- and loud. The photography seemed subdued, considering this is Barbados. There's little sense of sunshine and beaches. Though we see some sand and sea, they lack dazzle.
I think, if I had to choose, I'd prefer Hercule Poirot. He has a good many quirks that make him a person of some interest, while Miss Marple has only her knitting. It's also more fun -- and more politically correct -- to make fun of a short, squat, vain, finicky epicurean. It's harder to have fun with a little old lady who is essentially a passive observer.