As a Christie connoisseur, this saddens me whenever I hear of someone coming to this story for the first time via this particularly mediocre adaptation.

The best one, for my money, is the 1974 Sidney Lumet adaptation starring Albert Finney, which has a genuinely creepy opening and a far more sinister revelation of the murder. A much, much better film.

Also, I strongly recommend reading The Murder of Roger Ackroyd. The book is a gem that can never make a fully satisfying film for reasons that will obvious once you've read it.