One morning he came down [from his apartment], walked through the crowd, picked up a telephone and put it down (to match a studio close up), then walked over to the camera and looked through it to see what the outside line for his walk would be. [A crew member] said to Hitch, "You haven’t even said 'Good morning' to Cary. How does he know what to do?" Hitch answered casually, "Oh, he’s been walking across this lobby for years. I don’t need to tell him how."
Of course, Grant's Roger O. Thornhill lived elsewhere. But George Kaplan "occupied" room 796, which didn't exist - room numbers topped out at 769. And it is in this room that Roger underwent his existential crisis: he realized that he could easily be mistaken for Kaplan, that he was, as Murray Pomerance says, "Kaplanesque." So let's get this straight. In this scene, Cary Grant played a character who has been mistaken for another fictional character (an expedient exaggeration, as Thornhill would say), who is registered in a room not unlike Grant's in the same building. Okaaaaaaaay.
After a $400 million dollar renovation and three years under the scaffold, the Plaza Hotel has reopened, and about three quarters of its rooms have been converted to condominiums. Now, anyone with enough money can live like Grant did. If you want to experience happy hour like Roger O. Thornhill, however, you'll have to wait a bit longer. The Oak Bar, where Roger was kidnapped is still undergoing renovation and won't be open for another month or two.