Leonardo Dicaprio couldn't put on as much weight as he wanted to play J. Edgar Hoover.

The 'Inception' actor plays the historical figure in the forthcoming Clint Eastwood-directed biopic about the Federal Bureau of Investigation founder, and he admitted as much as he wanted to put on the pounds, he had to rely on the costume department for the most part.

He said: "I really wanted to take a break and do the whole real physical transformation. Of course, the film wouldn't allow it.

"It was a massive amount of weight to put on in a very short amount of time. So thankfully, I had a lot of people to help me in that transformation and I think it turned out pretty well."

Leonardo - who is known for his gritty roles in movies including 'The Beach' and 'Shutter Island' - reveals he was delighted to play Hoover because he helped shape the modern US.

He told Access Hollywood: "It was an honour to be able to play a character that has so many eccentricities, somebody that was in power in our country for over 50 years, through eight different presidents, who was in the underground of shaping the country that we know today.

"I didn't agree with a lot of his politics, especially later on in his career. But certainly the world he came into, he transformed our country. He was a true patriot in that sense."