I think, for me, this failed because I can’t imagine giving in to blackmail. When Sandy is blackmailed by the private eye in Britain, when he shows the envelope of money (containing £500) there is no way the blackmailer would mistake that for £10,000 in small bills.
And, while the premise, strangers on a plane discuss the movie Strangers on a Train, and think about how they can implement this in their real life situations, is good, it certainly isn’t Hitchcock!
I think the characters were inherently flawed. Sandy suddenly changes from being a womanizer to getting married in short order. Please! And Cara, just coming out of a terrible marriage, just jumps straight in again. Again, please.
Sorry, this just didn’t make sense to me.