Benicio del Toro plays an American Indian who undergoes a blend of psychotherapy and anthropological study in "Jimmy P."
The film, based on a real story, is competing for a prize at the Cannes Film Festival.
The movie is about "one human being getting close to another human being" without race playing a role, Del Toro said.
"The story of the American Indians is a great story that has still not been told and it is essential for understanding the person" of James Picard, a World War II veteran who undergoes innovative treatment in the United States in the 1950s, the actor said.
French director Arnaud Desplechin brought "Jimmy P." to the big screen.
The film, however, has received a cold reception at Cannes, where its long scenes full of dialogue and focus on psychoanalysis left reporters questioning its ability to win.
Del Toro and Mathieu Amalric, who plays anthropologist Georges Devereux, engage in long, intimate conversations.
The film marks Del Toro's return to Cannes five years after he won the best actor prize for his role in Steven Soderbergh's "Che."