According to Armie Hammer, Leonardo DiCaprio and he proposed to producer and director Clint Eastwood to depict the sexual relationship between the characters as graphic, but he refused, arguing the screenplay didn't call for it.
There was actually a huge fight between J. Edgar Hoover and Clyde Tolson in a hotel during one of their trips. According to producer and director Clint Eastwood, there are several testimonies of maids that talked about the room being destroyed after the discussion. However, screenwriter Dustin Lance Black dramatized the content of the argument.
Armie Hammer (Clyde Tolson) is the great-grandson of Occidental Petroleum tycoon Armand Hammer. In his biography of Hammer (the tycoon, not the actor) called "Dossier: The Secret History of Armand Hammer", author Edward Jay Epstein reported that the tycoon had a multi-decade history of being scrutinized and suspected of Soviet ties by J. Edgar Hoover. Armie stated in an interview that he took the role to avenge that scrutiny.
Leonardo DiCaprio and producer and director Clint Eastwood had a falling out during shooting. When Leo requested a retake, Clint called it a wrap and walked away. Both were cool to each other for the rest of the movie.
When J. Edgar Hoover died, he left his $551,000 estate to Clyde Tolson. He left Helen Gandy a separate bequest for $5,000.