Alfred Hitchcock was not a humble man. In Hitchcock, he and Truffaut rightfully come to the obvious realization that Hitchcock is alone atop the “suspense” genre. (Whether suspense is a genre is up for debate. When asked the question, Guillermo del Toro correctly observed that suspense is a device, not a genre. )
One could spend countless hours discussing and examining Hitchcock’s mastery of the “suspense” device. Truffaut cuts right to the chase, and rightly identifies one crucial aspect:
F.T. In several of your pictures, I’ve noticed, you will enhance a surprise situation with an additional twist; in other words — and I’m not thinking of only Psycho — you will use a bit of trickery to create a small suspenseful diversion so that the surprise that comes immediately afterward is even more startling.
A.H What do you have in mind?
F.T. Well, in Strangers on a Train, Farley Granger agrees to kill Robert Walker’s father, although, in fact, he really intends to warn the old man against the son. So Granger breaks into the house at night; the father’s room is upstairs. Now, if he simply tiptoed up the staircase, the public would try to figure out what’s going to happen next, and they might even guess that upstairs Granger will find Bruno instead of his father. So you dispose of that anticipation by creating a suspenseful diversion in the form of a huge dog in the middle of the staircase. In this way the question becomes: Will the dog let Farley Granger get by without biting him or won’t he? Isn’t that right?
A.H. Yes, in that scene we have first have a suspense effect, through the threatening dog, and later on we have a surprise effect when the person in the room turns out to be Robert Walker instead of his father. I remember we went to a lot of trouble getting that dog to lick Farley Granger’s hand.