Doing It for the Art: Nigel Hawthorne, inexperienced in cinema, took his role to prove that he had screen presence for the producers of The Madness Of King George. Hawthorne wanted to reprise the stage role for the movie version. As it transpired this was unnecessary as Hawthorne was the producers' automatic choice for the lead.
When Stallone got on board, he tried to convince Jackie Chan to take the part of Phoenix, which would have made Huxley's Saw It in a Movie Once moment a whole lot funnier. He turned down the role, as he didn't want to confuse audiences by suddenly appearing as a villian.
These casting choices would have made more sense when the film was intended to be a more straight action film rather than the satire it eventually became.
Edgar Friendly was originally known as "Thomas Payne" in the original script, which would have made a hell of a shout-out considering the character.