The dark family comedy centres on three World War II veteran brothers and their struggle to connect with their cold and cruel parent, Jim Caldwell.
Thornton directs and stars in the project he wrote, as one of the siblings, and the Bad Santa star admits he based Duvall's character on his own emotionless dad, William Raymond.
Speaking to reporters at a press conference following the movie's premiere at the Berlin Film Festival on Monday (13Feb12), he said, "My father was a very violent Irishman and so there was abuse both verbal and physical in our household. He was a Korean war veteran in the navy and he was a very intense guy who I don't think I ever had a conversation with."
And scriptwriter Thornton reveals scenes in the film, in which Duvall takes his grandson to car crashes to survey the wreckage, are also based on truth.
He adds, "He (my father) would take my brother and I... to car wrecks and he would stand there and smoke Lucky Strikes (cigarettes) and stare at the car wreck for two hours while my brother and I were like, 'Why are we here?' That was how he connected with us.
"Through all of that, through beatings and no communication or anything when I grew up, I realised that I understood my father and I loved my father."