Thoughts on Hitchcock's Use of Different Staring Scenes

During our class today, Mr. Hart told us that Hitchcock went into the editing room and used various scenes of Jeffries staring out the window, but not always the scenes that were filmed for that instance. For instance, take a scene of him actually staring at the lovely ballerina and use that as a scene of him staring at the Thorwalds. Mr. Hart suggested this could be so because a) Hitchcock really did not care for the scenes or if they were right, or b) Hitchcock really did not care for the actors' particular abilities and changed them. I had a different idea I wanted to share: Perhaps Hitchcock cared so much and was concerned that the true scenes betrayed too much emotion. I have no information to back this up with this film yet, but I'd think typically actors seem to be given their entire script --or at the very least know the plot of the film-- before filming. If this is so, then Jimmy Stewart would know that he's supposed to be staring at a future killer, and he may have accidentally betrayed some microexpressions that could affect the whole Russian Film Experiment aspect Hitchcock was clearly going for. Or, moreover, perhaps Hitchcock thought this could happen. From what little I remember reading about Hitchcock, he wasn't the most orthodox director. Perhaps he thought something could be revealed, and took the seemingly necessarily precaution against it: using different scenes. At this point, if such is true, Hitchcock is not only manipulating the viewers with the Russian Film Experiment itself --having each of us project our own thoughts of the events at hand onto the blank expressions of Jimmy Stewart staring out the window-- but also his actors by using scenes they did not intend for various other scenes.