In Hitchcock's North by Northwest the main character (played by Cary Grant) is an advertising man who gets thrown out of his dull life by first being kidnapped and almost killed and then wrongly accused of a murder, all this putting him on the run from both his kidnappers and the police. Each of his attempts to bring some light into what's going on only gets him entangled more deeply in the strange affair that he stumbled into by accident.
Well, what is going on is that Grant's character is mistaken for an intelligence agent; an agent, however, who doesn't exist, but whose identity was set up as a decoy to divert the attention of a spy ring from a second agent (this time a real one). We're told exactly that in a scene right after the first act (the first act containing pretty much what I've summarized in the paragraph above). A group of US Intelligence people around their mastermind, the 'Professor', discusses the situation, and we get the feeling that the whole scene's only function is to brief us, the audience, about every single bit of the background (with the exception of the identity of the other agent; to establish that is the main theme of act two).
What is the purpose of this comparatively undramatic revelation? Why tell us these things, instead of keeping them secret for as long as possible? The script could have been silent about them for much longer: they might have been revealed only at the time when the main character learns about them, preserving much more of the mystery.
Yet that's not Hitchcock's strategy. North by Northwest has this in common with Vertigo, another film where the main character gets drawn into an elaborate game of deception. And once more, there is a dedicated scene that simply reveals the plot of the mastermind behind that deception, in detail, and without inner necessity that would enforce the disclosure at the moment at which Hitchcock puts it in the movie. This time, it's the female lead (Kim Novak) who explains the background of what has happened so far by writing a letter to the main character and target of the deception (James Stewart), revealing the motive and method of the crime as well as the manipulative means she and the mastermind have employed. The letter never reaches the intended receiver, of course: it's just a plot device that provides a pretext for explaining something outright to the audience, without using any dramatic or cinematic means.
What this suggests, of course, is that unraveling the elaborate deception is not the main game the director had in mind for us as the audience. These movies are not about gradually getting behind the devious plotting of some mastermind; they're about something else, and therefore the mystery is quickly and thoroughly disposed of before it can get into the way too much. On the other hand, however, there is an elaborate deception going on in these films, so there must be some function to it. Otherwise, Hitchcock could just have left it out. But he does take some care to build it up. We're thus faced with the double question: what, if not the unraveling of the mystery, is the main way in which the director wants to engage us? And what, if it is not the riddle he wants us to solve, is the function of the complicated deception the main character undergoes and gets only gradually behind? (In contrast to the audience, the main character doesn't learn about the background all at once, and early on; the main character has to get behind everything painfully and slowly, and in the case of Vertigo he's in a tragic predicament which doesn't have any possible happy solution, so that he doesn't even get behind it completely.)
One possible interpretation is that Hitchcock's goal is to demonstrate the sort of struggle a character goes through in the face of elaborate unreality. That is what both films roll out before our eyes, in different constellations. Subjected to a complicated deceptive plot, they have to find ways to cope with events that don't fit their world. Getting behind the deception is part of their adventure; but it's not just an uninvolved interest they have: in both cases, their own lives and persons are threatened at a deep level. (And in both cases it's not a choice they've made on their own to embark on the sort of adventure they get in.)
In the process, the unreality (the fake world produced by the deception) gains a certain weight, so the hero's goal is not reversal simple and pure, but something that includes both getting back to reality and keeping something from the unreality. The main character in North by Northwest won't be going back to his "dull life", as he puts it; and above all he has met his future (third) wife in the course of the adventure (Eva Marie Saint). The main character in Vertigo is fully broken by the loss of the fake person he has met and the powerlessness he experienced when he failed to save her (a failure that was of course meticulously arranged for by the deceiving party), and he is unable to get into any meaningful relationship again because he compulsively tries to force the moment of crisis back from the past into the present again. Something has been created that cannot be reversed; even though it was unreality, it has caused real people to take real action, and has produced a new state of affairs (with both good and bad aspects). Unreality, one might, is no less of a 'real' force in the world than reality.