[North by Northwest] is arguably as good an indication as can be found of Alfred Hitchcock's ability to match his sense of humour to his lively suspense tactics. Strictly speaking, according to those who claim to know such things, the title is a compass point that doesn't exist. It is our cue to disregard any measure of 'reality' in the film. 'Realism' is there, of course; but not, overtly at any rate, 'reality'. Further nudges towards levity are given by the leading character, Roger O Thornhill …, who emphasises that his initials spell 'rot', and that his central 'O' stands for nothing. There is, however, a nuance to that 'nothing': the plot is one of mistaken identity, Thornhill being considered by the villains to be a spy who is actually as non-existent as the compass point to which symbolically he eventually travels; and therefore when this false identity is...