Martin Scorsese has created some of cinema's most unforgettable criminals, but none feel quite as unpredictable as Frank Costello in The Departed. While the screenplay laid the foundation, it was Jack Nicholson's fearless improvisation and on-set rewrites that transformed Costello from a traditional mob boss into one of the most unsettling villains in movie history. Jack Nicholson's performance in The Departed remains one of the greatest examples of controlled chaos ever captured on film. Every scene feels dangerous because no one, including the audience, knows what Frank Costello will do next.