I've not seen the 1934 original, so I can't compare this 1956 remake with the earlier version.
The tension is certainly there in true Hitchcock style, especially in the Albert Hall assassination scene, but somehow the film seems wooden and dated.
I can forgive it being dated - after all, it was made 45 years ago, and tastes and production techniques change: maybe artificial-looking back-projection really was the only way to film the scenes inside the bus and on the horse-drawn carriage straight afterwards. The London streets are far too quiet: where are the pedestrians, the traffic and the parked cars? And why do the aerial shots of the Ambrose Chapel look so obviously like a film set?
However the wooden, underplayed response of Ben and Jo McKenna to the kidnapping of their son doesn't ring true and is less excusable: where is the anguish and despair? All we get is Jo McKenna's over-the-top reaction when she finds out that Hank has been kidnapped; why does Ben McKenna not seem to be at all perturbed? Having said that, Hank McKenna must be the most insufferably precocious child, even by American standards of insufferability and precocity!
There are two plot holes which are never explained: how do the British police (Buchanan and Woburn) know that Hank has been kidnapped as soon as Ben and Jo arrive in London, since they have told no-one? And how does Jo know that someone is going to be assassinated at the Albert Hall and when it's going to take place? All she knows about the Albert Hall is that it's where she'll find Buchanan.
The most memorable character by far - and it's a truly spine-chilling performance - is that of The Assassin: Reggie Nalder is superb, especially his sinister smirk when he tells Jo about Hank in the foyer of the Albert Hall.
The film includes a couple of good lines:
- When Hank and Louis Bernard are talking about eating snails, Hank quips "If you ever get hungry, our garden back home is full of snails. We tried everything to get rid of them. We never thought of a Frenchman!"
- When the Ambassador is reprimanding the Draytons, he ironically says "You have muddled everything from the start, taking that child with you from Marrakesh. Don't you realise that Americans dislike having their children stolen?"
Having said this, "The Man Who Knew Too Much" is a good story, told well. It's only in comparison with Hitchcock's other films that it suffers. For a good Hitchcock film, see "North By North-West" or "Frenzy" (his best, IMHO).