Cinema and Television Verities #37:

“You know that the public always likes to be one jump ahead of the story; they like to feel they know what’s coming next.  So you deliberately play upon this fact to control their thoughts.  The more we go into the detail’s of the girl’s journey, the more the audience becomes absorbed by her flight.  That’s why so much is made of the motorcycle cop and the change of cars.  When Anthony Perkins tells the girl of his life in the motel, and they exchange views, you still play upon the girl’s problem…”

- Alfred Hitchcock discusses Psycho's "Janet Leigh trick" with Francois Truffaut, in Hitchcock, (Touchstone Books, 1983, Page 269.)