Wilder had a soft spot for murder mysteries - Double Indemnity, Sherlock Holmes, this... Here he took Agatha Christie's first-rate play source material and expanded it - beefing up the part of barrister Sir Wilbur... not giving it any great emotional depth but adding a comic subplot where he squabbles with his nurse who wants him to give up cigars and brandy. This was presumably done to attract Charles Laughton.
He also adds two flashbacks involving Leonard Vole, one where he meets his wife. Presumably this was to give more time to Tyrone Power and Marlene Dietrich. The dialogue of their first meeting - cynicism in post war Berlin - feels the most "Wilder" out of any in this script. (Having said that it'll probably turn out that someone else wrote it!) None of this stuff is essential to the story - it's quite a long read - but it does add to the fun.
It's a satisfying mystery, based on the clever concept that people are more likely to believe a woman if they think she's been lying, than someone telling the truth. I've never believed in a core twist... that no one would think the cockney woman was Christine.
The script is written so that towards the end there's a page which just says "the last ten pages are secret".... I read the rest, but apparently it was issued separately. It's a clever idea. Civilised entertainment from Wilder.