This must be the first tension drama in which a murder is planned to happen at a particular moment in an orchestral performance. I have seen many similar variations since. The film has a familiar Saturday morning feel to it. Good enough to pass the time but not breathtaking. Doris Day has a reasonable singing voice but Que sera sera is not good enough to be sung twice in one film. Hitchcock uses the English actors to play the clumsy villans, the French to play the immoral murderer and the Americans to play the Hero/Heroine. If you stomach the stereotype it is watchable.
One of those - "what would I do in this situation"? films where no-one really believes what is going on when trying to explain with a bare minimum of facts and only a hunch as to what the situation may develop into. A vehicle for both Stewart and Day to show that they can act (and in the latter's case, sing), with scenes in several countries (albeit as an obvious film backdrop in some scenes) but overall a very good film with a clever backdrop of how the murder will be attempted, as stated, possibly the first time that this 'cover' has been used cinematically.
It's difficult to watch this film without comparing it to the version from 1934. And it's not all bad. James Stewart and Doris Day dominate the remake in a way that Leslie Best and Edna Best weren't expected to 22 years earlier. And Day's mental fragility is a good addition.
Some of the negativity that gathers around the retread isn't really a fault; the original was a breakthrough film by a director on the cusp of greatness, the second version is the least of his stunning sequence of fifties thrillers. He owed his studio a film, and he considered any of his British films for an update.
The best scene is still the finale at the Albert Hall. It is expanded to a considerable longer running time, but the suspense still holds. Composer Bernard Herrmann showed good judgement in retaining the same music for the climax as in the 1934 version.
Though the 1956 picture has a bigger budget, finer gowns and 45 minutes of extra running time, these advantages don't amount to a better film. And that comparison makes it feel bloated and indulgent, especially during the long new ending at the embassy. It's still a decent work of suspense. Its misfortune is that the Master had already made it so well before.