What makes Orson's entrance so effective is that he doesn't do an entrance. He just stands there in the shadows, lurking ominously and making Joseph Cotten uneasy. We see his face only because an annoyed Austrian turns on the light. Thus Orson's entrance is that of a rat when we open the manhole cover to the sewer. And then he runs away back into the sewers.
Joseph Cotten discovers the man's presence and takes him for a spy. Why would someone spy on him, he doesn't know, but nothing will surprise him after what he already had seen during his brief visit to Vienna. He calls the spy out, but he's cautious. He's comfortable with calling him names from a safe distance, but won't make an attack himself, bluffing to scare the man away. But it doesn't work. The ominous shadow doesn't move at all, doesn't show any signs of nervousness at being so suddenly discovered. He's obviously very sure of himself, so why attack and risk a bump on the head?
And then suddenly Joseph discovers that the man is his [spoilers] lost friend that he was told was dead. His world suddenly falls apart. The streets tilt and the parked cars roll down. But he doesn't care. He has to get to the black doorway to make sure it really is Harry. But the shadow runs away. He plays a trick on him but won't get caught to reveal the mystery.
