Totally agree on Why Didn’t They Ask Evans?. Absolutely textbook example, IMHO, of the right way to embellish and “update” (and even somewhat—gasp!—diversify!) a classic period piece.
Respect the source and the period in your use of dialogue (which doesn’t mean you can’t get away with inserting a few swears, of the sort that many interwar-era people would have actually used even if an Agatha Christie novel would never explicitly record them).
Make a script that requires actual acting and—[clueless golden retriever trying to understand movies] what’s it called in film stuff when you can see something about the characters and their interaction just from non-plot-driven non-verbal behavior? Interpretation? Business? Directing? Shot composition? Visuals?[/cgrttum] Not just reciting sentences of plot exposition and provocative rhetoric hamhandedly trying to convey either Gee People Were Just Terrible Back Then or This Admirable Character Somehow Miraculously Understands What’s Bad About How Terrible People Were Back Then.
Similarly, make the modification or “diversification” of supporting characters an opportunity to introduce some interesting people who make the story more interesting. Not just putting new cardboard masks and hats onto the original cardboard personae of a formulaic mystery novel.
Use the comedy that’s in the source material, rather than trying to get your laughs by clumsily lampshading and mocking How Silly And Prudish And Unnatural People’s Manners Were Back Then. Agatha Christie is actually often a very funny writer, in spots, in an understated comedy-of-manners way, and the show gave space to that and built on it.
IIRC I originally viewed it in six separate installments, so maybe they bundled them?