Leonardo DiCaprio said J Edgar Hoover has so many "eccentricities" that they couldn't fit them all in J Edgar the film.
The actor plays the late founder of the FBI in Clint Eastwood's upcoming movie, and said it was impossible to portray every element of his complex character on screen.
In a video posted on The Hollywood Reporter, he said: "He really was a crockpot of eccentricities, we couldn't even fit them all on screen. The man lived with his mother until he was 40 years old, he never had a personal relationship of any kind, whatever happened behind closed doors no one ever got to see."
Leo added that he had visited the Hoovers' home, spent a day at the FBI Headquarters and met one of his former colleagues to research the role, and spoke highly of Eastwood's style of directing.
He said: "He's a director that relies on his own instincts and pushes you as an actor to trust your own instincts because as a director he trusts his."
Dustin Lance Black, who wrote the screenplay, described his motivation for writing the script, saying: "I thought what a shame we don't truly understand the man who shaped this country."
:: J Edgar is out in cinemas on 20 January 2012.