Clint Eastwood has cast Naomi Watts in new John Edgar Hoover biopic 'J. Edgar'.

The Australian actress is to play the FBI director's secretary Helen Gandy, who worked for him for 54 years.

After he died Gandy was responsible for destroying his personal file which contained incriminating material that he used to control powerful political figures.

Watts will star opposite Leonardo DiCaprio, who is playing J. Edgar, in the movie, while the cast also includes Judi Dench as Hoover's mother Anna Marie and Ed Westwick as Hoover's biographer Agent Smith.

The 42-year-old actress was up against Charlize Theron for the role but after Theron decided to accept the part of the Evil Queen in 'Snow White and the Huntsman' scheduling conflicts meant she lost out on the part.

Hoover was the first Director of the Federal Bureau of Investigation (FBI) of the United States and was an instrumental figure in forming the unit in 1935. He personally directed the FBI investigation into the assassination of President John F. Kennedy.