Hollywood, CA (WFN) -- Portraying mentally-troubled aviation tycoon Howard Hughes in the upcoming movie, "The Aviator" was no walk in the park for star Leonardo DiCaprio.

To accurately depict Hughes' struggle with Obsessive Compulsive Disorder, the actor consulted an expert from UCLA's Neuropsychiatric Institute -- but the research caused some of Leo's own OCD behaviors to resurface.

DiCaprio says he reverted back to a time in his childhood where he developed some strange tics, like retracing his path just to step on a crack he'd passed earlier.

He says his repetitive behavior "really kicked in full force when I let it." It sometimes took him 10 minutes to make the short walk from his trailer to the set because he kept stopping to check things.

DiCaprio's technical consultant, Dr. Jeffrey Schwartz, isn't surprised the actor became a little obsessive while playing Hughes.

He says brain imaging studies show that when actors depict mental illnesses, their serotonin levels and blood flow take on the same characteristics as the patient they're portraying.

"The Aviator" opens in theaters Dec. 17.

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