A book on phrenology reads about the science that studies relationships between a person's character and the morphology of the skull.
The "Titanic" star was immediately drawn to the role of ruthless plantation owner Calvin Candie and he wanted to aid Tarantino in developing the character so he presented him phrenological book, reported Daily Star.
"Writer-directors tend to be very precious about their material and their words, but Quentin's whole process is getting input from the actors and adding levels to their characters," said DiCaprio.
DiCaprio recently said he was nervous to portray Candie as a brutally violent slave master and considered softening his character, until he had a word with co-stars Jamie Foxx and Samuel L Jackson, who both play slaves.
"One of the pivotal moments for me was this initial read through, I wondered if it needed to be this violent and this atrocious to other human beings and it was Sam and Jamie who said, If you sugarcoat this people are gonna resent the hell out of you. You got to push this guy to the outer extreme," he said.
"That is what ignited me into going the way I did into the character. Once I read about the sugar plantations, we're just scratching the surface of what happened in this country," he added.