London, February 9 : Leonardo DiCaprio studied the mentally ill and watched several dark documentaries to perfect his performance in the new thriller Shutter Island.

The Titanic actor took to research to master his role of a detective who tries to solve a missing persons mystery in an asylum.

"There was a tremendous amount of research done involving the entrapments of mental illness and the suffering that people go through, so it led me to watch a lot of different documentaries on mental illness," the Daily Express quoted him as saying.

"For a few weeks there, it was some of the most hardcore film experiences I've ever had. It was like reliving trauma in a way. It was pretty intense," he added.

The star further continued: "We were surrounded by it (mental illness) every day. We were surrounded by dilapidated walls of an old mental institution. We had somebody there who was guiding us through the history of mental illness, the pathways of treating it and the different forms of treatment."