I know this topic is very old, but I couldn't help responding. I've recently gotten interesting in writing cozy mysteries (though I've loved reading them and watching films and TV series of them for years) and am now trying to read and watch as much of them as I can to educate myself as to how they work and what makes them tick. So I've started with the classics (Agatha Christie, of course, and Sherlock Holmes), rereading them and watching films and TV shows so I had to put my two cents in.
As for Poirot, I've seen most renditions of him on film and TV so far (I think). I didn't really like Albert Finney's portrayal of Poirot (I felt that Poirot as Christie created him is a little uncouth by English standards, but Finney made him downright aggressive). I thought Peter Ustinov was on the right track (and Ustinov is such a brilliant actor, nothing he ever did could be ruined) but I agree with many people that his sheer physicality was a bit too much for the Poirot character (since Poirot is supposed to be a "funny little man").
To me, Suchet nailed it, giving the character both elegance and grit. Suchet, like Hickman, actually also had a part in a Christie film that I just saw a few days ago - "Thirteen At Dinner" (1985) with Faye Dunnaway. Ustinov was Poriot and Suchet played Inspector Japp. He's far better as Poiriot, though.
As for Miss Marple, I also think Geraldine McEwan does a nice job (though I've read that many people don't agree with this). She's believable as the "dotty old maid" who can solve crimes. I actually didn't like Joan Hickson much (though I'll admit that I only saw one of the shows in the series). I thought she was too passive and a bit too abrasive to play "a dotty old maid" who could solve crimes. I like Margaret Rutherford as an actress a lot, but not so much as Miss Marple - I think she's a bit too gitty for it. BTW, the comment someone made her about Dame Agatha not liking Rutherford as Marple is true, but I also read that she dedicated one of her Marple novels to her (can't recall which one).
Also, some might not know, but Angela Lansbury also played Miss Marple for one film - "The Mirror Crack'd" (1980) with Elizabeth Taylor and a host of other well-known stars. I thought she did an awesome job. I read she was slated to do some others but it didn't pan out (but then, we got her as J.B. Fletcher in "Murder, She Wrote" for 12 years, so the cozy mystery genre didn't neglect her talents there :-D).
I'm actually surprised that the film industry seems to have revitalized the Sherlock Holmes character in film in the last few years (though I haven't seen any one of them so I can't say how good they are or are not) but Dame Agatha's characters are showing up only on TV. Any guesses as to why?
Djuna