I had read POF years ago, but I recently reacquainted myself with the story via Hugh Fraser's audio book version.
I became very excited by it and---just for the fun it--began making notes for a screen play.
I would say Hugh could play Tommy, but I think Christie fans wouldn't accept him in another Christie role besides Capt. Hastings. (same as David Suchet would make a great Paravacini if they ever filmed MOUSETRAP, but that probably will never happen).
Not sure who would play Tuppence.
The book, although not her best, is very adaptable! Lots of room for flashbacks and filling out the unexplained points in the book. The conclusion seems very much like Passenger to Frankfurt but that made a bit more sense overall.
POF seems to have needed one more edit on the last chapter to explain what exactly happened to Mary Jordan. The main clue that T&T find doesn't really go anywhere.
But of course my screenplay will change all that.... LOL