The Spanish Prisoner

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.

If you watch the scene in the men's room in Central Park with the group of people alleged to be the FBI, Joe Ross (Campbell Scott) is never shown taking his eyes off the book the formula is recorded in. The ersatz FBI agents could not have switched it for the blank book, and yet, Joe was looking through it just before they met and the pages were not blank.

Watch carefully when Joe hands the book to the phony FBI Team Leader (Ed O'Neill). It's clear that Joe never looks away from the book while it is out of his hands. The Team Leader leafs through the pages, and sets up on a high window ledge while Joe puts his coat back on, but Joe never lets it out of his sight until the Team Leader hands it back to him. The imputation is that the switch happened in that meeting, but it could not possibly have happened.