First, if you haven't seen the Martin Scorsese short film based off of an Alfred Hitchcock script, do so. It is really well done. Second, if you found that interesting you might find this Slate slide show on the myth of Alfred Hitchcock being involved in every aspect of his films interesting as well. Most of it is 'well duh' type stuff but there is a least one new to me thing.
On at least one occasion, publicists resorted to trickery to burnish Hitchcock's reputation as a master planner. The Block exhibition catalog, Casting a Shadow, contains a revealing memo regarding North by Northwest. "I'm sending about 13 stills from which I would like Mr. Hitchcock to make the sketches I discussed with him before he left for Europe," wrote Hitchcock's publicist, Rick Ingersoll. "As a reminder, they are for Coronet magazine—and theoretically sketches he made before the scenes were filmed. This is for a layout in which his sketches and the resultant scenes would be compared, to show how he maps out every detail of his productions before the scenes are photographed." Hitchcock may have made precise sketches of his scenes, as his Times obit claimed—but apparently not always before he began filming.