For Hitchcock the important realities were always mental ones.
Unfortunately, much of his book is written in the often impenetrable jargon of lit and film “theory,” which makes it a chore to get through—in addition to imperfectly disguising the unremarkable quality of many of its critical insights.
For the more strictly cinematic sort of genius which everyone agrees Hitchcock had, the best bets are the volume edited by Gottlieb, and the exploration of the lesser-known byways of his long career, , by Alain Kerzoncuf and Charles Barr—though the latter, consisting so largely of piecing together bits of information about films that no longer exist, is likely to be of more interest to Hitchcock scholars and biographers than a general audience.
Indeed, this particular sequence was probably completed before Hitchcock came to work on the film. Sometimes the name alone will effect the change, and our minds do the rest.
Perhaps he intends by this observation to adduce a hitherto unknown corollary of the Kuleshov effect, one of Hitchcock’s favorite film-making principles, which explains how cutting or montage—context, as the non-film-making world would describe it—affects what we see, and how we understand what we see.
To Hitchcock there typically isn’t a real trail—or not one that matters any more than a false one for its own sake.
Everything matters only subjectively, through its effects on the main character—and through him or her on the audience—whether the knowledge is true or false.
Or rather, he makes a claim to knowledge to which he is not reasonably entitled solely on the grounds of his reverence for the allegedly iconic Hitchcock’s genius.
What he knows or thinks he knows is of less interest or importance, either to him or to us, than he knows it—merely through his knowledge of Hitchcock’s participation in the film.
And even when the apparently wrong man becomes the right man, as in (1950), the audience’s surrogate (a vulnerable young woman in both cases) must be taken in by the deception along with the audience.
Either way, it is the audience’s latent paranoia he is always appealing to.