"There are actors who play kings, and actors who do not play kings. I am one of the actors who plays kings."
Welles forgot this only once, when he played seaman Michael O'Hara in The Lady from Shanghai, and his performance (for once in his career playing a fairly traditionally heroic part) is one of the weaker aspects of this film. It was originally supposed to be a "B" picture, but then Rita Hayworth (then married to Welles) decided to star in it, so it became an "A" picture after all.
The villains (pretty much everybody else in the picture) are excellent, especially Everett Sloan as Bannister the famous lawyer and Glenn Anders as Grisby, Bannister's partner. The story is a thick soup of plots and double-crosses, and Welles' character is the only one without even a single hidden agenda.
The film is rather uneven, with some of the scenes being very powerful, but the stand-out moment is the (very famous) shoot-out in a house of mirrors that ends the film.
That some of the aspects of the film that are not appropriate for Welles is perhaps explained by the fact that he convinced Harry Cohn (President of Columbia Pictures) to make the film (and to let him direct it) when he needed to have Cohn advance him some money he could use for another project, and he had never read the book (Sherwood King's If I Die Before I Wake). It just happened to be on the table in front of him when he placed the call.
At least this is the story that Welles told later on, and it's therefore at least somewhat suspect.