David Mamet cast Steve Martin in an atypically villainous role after seeing him perform in Waiting for Godot on stage. He felt instinctively that comedians can make very strong dramatic actors as Jackie Gleason proved in The Hustler (1961) and Jerry Lewis did in The King of Comedy (1982).
Mamet wrote the part of Jimmy Dell specifically for Steve Martin in order to take full advantage from the comic playing against type. He was one of the first to recognize that Martin, renowned for his manic energy, possessed a deep well of seriousness which allowed Martin to portray his character as calm and in charge, which in turn made him appear menacing.
When he is hung over, George Lang says, "I put a thief in my mouth to steal my brains." The line is a close paraphrase of one from Shakespeare's "Othello": "I would not put a thief in my mouth to steal my brains."
The very first shot of the movie contains the curative advice that could have prevented the con from being effective. Joe is seen at the airport standing under the sign that warns passengers to not take packages from anyone.
George Lang's line "Beware of all enterprises which require new clothes" is by Thoreau ("Economy" chapter of Walden).