Look Carefully at the Berlin Museum...

It's a special effect!

Because of political tensions between East and West, it was impossible to actually shoot the movie at the real Berlin Museum. The sets for the Berlin Museum are comprised almost entirely of matte paintings by Albert Whitlock http://www.imdb.com/name/nm0926087/.

From the book "The Invisible Art: The Legends of Movie Matte Painting" by Mark Cotta Vaz and Craig Barron:

Joe Musso recalls an exchange between the director and the actor: "In this shot Newman had nothing real to react to, and being a Method actor, he needed an environment. So he asks, "Hitch, what's my motivation for walking in on a straight line?" And Hitchcock says, 'If you don't you will disappear under the matte painting.'"