The Fair Game star has held off competition from Amy Adams to win the part of Hoover's personal secretary Helen Gandy. Charlize Theron was previously rumoured for the part but reportedly turned it down to star in Snow White and the Huntsman.
Gandy worked closely with Hoover, the first director of the FBI, for more than 50 years and destroyed files that allegedly contained confidential records following his death in 1972.
J. Edgar will star Leonardo DiCaprio as Hoover, while Ed Westwick, Armie Hammer, Judi Dench and Josh Lucas are among the supporting cast.
The movie is due to open in theatres in 2012.