Interesting: Saul, as far as I know, never used this technique before or later.
The title sequence consists of little paper flaps that close and open to reveal the credits.
You can see the shadows where they do not lie completely flat.
Always reminds me of one of those French boulevard stage farces where doors keep opening and closing
(also an almost stock cartoon item).

One of the few Billy Wilder movies where Saul Bass actually was credited.