The Dreamgirls star plays the wife of former South African president Nelson Mandela. The movie, Winnie, which also features Terrence Howard as Nelson Mandela, is set to hit screens later this year.
Oscar-winner Jennifer, 29, said of the biopic role: 'It was the hardest thing I've ever done. I can't wait for the world to see this. Her story is so powerful and amazing.'
Jennifer has previously told how she slept in solitary confinement in a prison cell for several days to prepare for the part.
Winnie Mandela has reportedly previously threatened legal action over the film, complaining that she was not consulted.
A lawyer for Winnie, 74, said in a letter: 'It is difficult to understand how a production bearing the name of an individual who has not been consulted at all could ever be appropriate or tell the full story of that individual's life.'