Navin is an idiot. He grew up in Mississippi as adopted son of a black family. But at his 18th birthday he feels he wants to discover the rest of the world and sets out to St. Louis. Everyone exploits his naivete, but then a simple invention brings him a fortune.
The juggling scene is over a minute and shows Steve Martin, as a
Mexican carnival performer, juggling 3 kittens. From the script:
(A scene in a Mexican bar - cat juggling!! Navin is shocked. The
juggler looks suspiciously like Navin, but rest assured, according to
the movies credits, Steve Martin plays Navin, and Pig Eye Jackson is
credited as the cat juggler.) "Good Lord - I've heard about this - cat
juggling! Stop! Stop! Stop it! Stop it! Stop it! Good. Father,
could there be a god that would let this happen?"