It's been a curious challenge for Leonardo DiCaprio on his new movie, "Shutter Island," requiring the star to research and understand mental illness.

In the drama, set in 1954, Leo plays a US Marshall investigating the disappearance of a murderess who escaped from a hospital for the criminally insane -- and is presumed to be hiding on the remote Shutter Island.

The film also stars Mark Ruffalo, Sir Ben Kingsley, Patricia Clarkson and Michelle Williams.

"The more we researched this movie and the more we started to uncover the different layers of the story line, the deeper we had to go," DiCaprio tells "Extra," adding, "It kept pushing us further and further and further."

DiCaprio said the cast had a doctor on-set to help the actors understand the point of view of someone who suffers from a mental illness.

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He says "Shutter Island" (in theaters Feb. 19) is a throwback to Alfred Hitchcock's "Vertigo." "But it's a Martin Scorsese movie, so it goes to even darker places."

The three-time Oscar nominee also tells "Extra" he's happily team with Scorsese again for the upcoming Frank Sinatra biopic. "Anything he calls me for, I'm pretty much down," Leo dished.

The 35-year-old is also busy raising funds for Haiti. DiCaprio donated $1 million and lent a hand during George Clooney's "Hope for Haiti Now" benefit last week.

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"The generosity has been pretty tremendous," the actor/humanitarian says. "It's one of the great disasters of our time and people need to keep giving."

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