"I was sizing him up the whole time," Jordan said, flashing a megawatt smile at Stevenson as they sat, reunited, on a September day in Toronto, earning an amused grin in return. "I thought I was doing it without him noticing, but he told me he kind of felt it."

Jordan, 32, known for his roles as Oakland police-brutality victim Oscar Grant in Fruitvale Station, villain Erik Killmonger in Black Panther and boxer Adonis Creed in Creed, plays Stevenson in Just Mercy at a formative time in the jurist's life, as an eager young lawyer whose calling is awakened as he represents condemned men and is confronted with injustice, corruption and racism.

Now 60, Stevenson is co-founder of the Equal Justice Initiative, a nonprofit that combats racial and economic inequity in the criminal justice system. In a 35-year career he's saved 125 prisoners from death row, including condemned men whose cases and traumas are depicted with sensitivity in Just Mercy. He speaks passionately and urgently, but also with a sense of optimism.

Yet before he became a prominent advocate in the justice reform movement, one who inspired others to action in a viral TED Talk, Stevenson was the idealistic young lawyer we meet in the opening scenes of Just Mercy, - inexperienced, under-resourced and unprepared for the adversaries and obstacles that come his way as he sets out to represent death row inmates, mostly poor minorities, in the deep South.

It is this Stevenson whom Jordan locates at a pivotal moment in his professional and emotional life. Just Mercy, directed by Destin Daniel Cretton (Short Term 12) and written by Cretton and Andrew Lanham, is based on Stevenson's 2014 memoir of the same name. It tracks his career circa 1988, when as a 28-year-old attorney he took on the case of Walter McMillian, an African-American man sentenced to die in Alabama for the murder of a white woman, despite his claims of innocence and dozens of witnesses corroborating his claim.

Through proximity to his first death row clients, Stevenson developed a deep sense of empathy for the humanity they were so often denied by the system. Most were disproportionately disadvantaged and many had no access to legal aid or were suffering under excessive punishments, largely discarded and forgotten by society.

His book became a bestseller but he realised that its message could reach even more people in film form.

"I just wanted people to see what I've seen for the last 35 years," Stevenson said in August in Los Angeles. "I've seen people crushed by inequality and injustice ... and I just believe that my reactions are not unique."

Before they met, Stevenson had been an admirer of Jordan's acting on The Wire and Friday Night Lights. "It was such a strong and human story," Stevenson said of FNL, "and I loved Fruitvale Station."

But it wasn't until the two sat down to get to know one another, after Jordan finished filming the Oscar-nominated Black Panther, that a bond grew between them.

To prepare for the role, Jordan traveled to Alabama to spend time with Stevenson at the Legacy Museum and National Memorial for Peace and Justice, founded by the Equal Justice Initiative to preserve and educate the public on the history of enslavement, lynching and racial oppression in America. He found himself not asking questions so much as listening to Stevenson.

"You're almost in awe of him when he speaks," said Jordan.

"It was more me just trying to soak up everything he was saying. Everything he says has so much weight to it and meaning. I was just trying to catalogue it all."

Stevenson was a resource for Jordan and Cretton as they recreated specifics from his cases and milestones from his early days, working out of a ramshackle office with collaborator Eva Ansley (Brie Larson), and portrayed the stories of McMillian (Foxx) and his fellow death row incarcerees Anthony Ray Hinton (O'Shea Jackson Jr.) and Herbert Richardson (Rob Morgan, in a standout supporting turn).

"Everything I needed to know was already there," said Jordan.

"It was identifying with our hearts being similar, and our spirits and our intentions and our morals aligned. It was cool to see a little bit of myself in him. And once that part connected for me, I felt a little more comfortable about taking on this challenge." Just Mercy opens a window into the impact the work had on the young Stevenson, but it also calls attention to the racism he was not immune to. Mirroring an earlier scene in which McMillian is pulled over, Stevenson is stopped by police. Like McMillian, he cautiously and instinctively places his hands on his steering wheel with the knowledge that any sudden or wrong move can result in his death.

It's a parallel moment that happened accidentally as Foxx and Jordan separately brought their own experiences to the film. Jordan said of the unplanned synchronicity, "Anybody that looks like me that has been pulled over by the cops - we know what that feels like".

"We know what that anxiety is, that nervousness. That wondering: Am I going to make it out of this? It's knowledge that you have to know as a black man, as a person of colour, going out into the world, that these are the rules that you have to abide by to get home safe. I think that's the shared experience between Bryan and Walter in those moments; it's like, we know the drill."

Just Mercy will be released on January 23, 2020.