How did Robert De Niro prepare for his role in casino?

Robert De Niro prepared for his role in "Casino" by immersing himself in the world of Las Vegas and the casino industry. He met with real-life figures involved in organized crime and gambling to understand their lifestyles and methods. De Niro also studied the nuances of his character, Sam "Ace" Rothstein, by learning about the intricacies of casino operations and the psychological aspects of his character's rise and fall. This thorough preparation helped him deliver a compelling and authentic performance.