The star of the award-winning film Clemency talks about the US prison system, her enslaved great-grandfather and her hopes for Black Lives Matter

• Read Mark Kermode’s five-star review of Clemency

The focus of Black Lives Matter protests has inevitably fallen on the most visible injustice - instances of police brutality. More systemic racial disparities in the American penal system are too often hidden from plain sight. The US incarcerates more of its citizens – 2.2 million people – than any other country on Earth. African American adults are nearly six times more likely to receive a prison sentence than white adults. Nearly half of the 206,000 people serving life sentences in 2018 were black, though black people represent only 13.4% of the population; almost equal numbers of white and black prisoners are currently on death row – just over 1,000 of each ethnicity – but as the prosecution of capital punishment has declined, so the racial imbalance has increased.

If ever a film could bring home the buried trauma of those latter statistics it is Clemency. The film, which won a grand jury prize at Sundance last year, has been instrumental in catalysing again urgent debates around mass incarceration, capital punishment and race.