Here is a statement from De Niro's co-star, Lily Gladstone, expressed on how much the actor dedicated his role:
"He was fully committed to getting it right, spending every waking moment on it."
Gladstone also revealed that De Niro's commitment extended to his sleep, as his partner, Tiffany Chen, jokingly claimed he would speak Osage even in his dreams.
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De Niro confirm Tiffany Chen's statement as he said:
"She caught me mumbling. I was constantly practicing and reviewing. I couldn't afford any mistakes onscreen. I had to truly know it."
Half a century ago, De Niro learned to speak Sicilian to play young Vito Corleone in Book-Inspired Film The Godfather Part II—a role that earned him his first Academy Award. However, he admits that learning Osage proved to be even more challenging.
Here is his statement about the challenges learning the Osage Ianguage:
"I had some familiarity with Italian, purely out of personal interest. I had a natural inclination towards it. However, tackling Sicilian was still a tremendous effort. I even traveled to Sicily, spending a few weeks there to record native speakers."
The book-based film "Killers of the Flower Moon" has received 10 Academy Award nominations, including Best Picture, Best Director for Scorsese, Best Actress for Gladstone, and Best Supporting Actor for De Niro, is currently available for streaming on Apple TV+.
Source: People