The actor was raised with his cousin, who lived in the Whitakers' home and shared a bedroom with Forest and his younger brothers, Kenn and Damon, until he was drafted in 1968.
Whitaker admits his unidentified relative was completely withdrawn when he came home two years later, and the wannabe actor felt desperate to understand what he had experienced.
Whitaker tells The Sunday Times Magazine, "When he came back, he wasn't the same person. I tried to talk to him about what happened to him in the war, but he didn't want to - I was still a kid. He never talked about it. Ultimately, he was consumed by alcohol."
Whitaker went on to star in Vietnam movies including Platoon and Good Morning, Vietnam as a way of exploring the effect of war on men on the battlefield.
He adds, "I was trying to understand what happened to my cousin during the conflict. In some ways, the films were my first little touch with war."