The 32-year-old Brazilian actor will star opposite Benicio Del Toro, who plays the legendary revolutionary in Che.
The much-anticipated movie is based on the diaries of the Argentine guerrilla Ernesto 'Che' Guevara.
Raul Castro replaced his brother Fidel as the leader of Cuba in February, and Santoro admits he knows his portrayal is "a delicate" one.
Santoro says, "As an actor, I have to try to incorporate the human being. I cannot do a political analysis about him."
But he's not surprised he won the supporting role in Che: "He (Castro) has slanted eyes. I think the producers saw that and thought, 'Look here, there's a great physical resemblance.'"
To prepare for the role, Santoro spent two months in Cuba last year, visiting places where the Castro brothers grew up, including the house where they were both born.
He says, "It was a very strong experience."