Everyone thinks Samuel L Jackson is about 15 years younger than he really is. It’s the hair, probably, or the absence of it. I think we’ve all come to accept that Jackson keeps a rotating carousel of different movie wigs somewhere at home, and that none of his movie hair is ever real. No steady progression from dark to grey to white means the ageing process seems almost to have halted itself, and the man before me today, shaven-headed, tall, enviably lean and energetic, talkative and affable, could pass for a fit 45-year-old. Except he’s 67.

Jackson’s latest role, in The Legend of Tarzan, is a real-life figure inserted into a fictional universe, George Washington Williams, who achieved things in his lifetime that one is shocked and pleased to learn were achieved by any black American in the latter half of the 19th century. For a historically minded man such as Jackson, whose teenage years coincided with the optimistic height of the civil rights struggle, and who was a young Black Panther in the bleak and treacherous COINTELPRO years, the role probes some unfamiliar backwaters of the African-American experience.

“At 14 years old he enlists in the civil war,” he says, “then he enlists in the Mexican revolution against the Emperor Maximilian. I don’t know if he was an actual Buffalo Soldier, but I know he fought in the Indian wars too, and he killed a bunch of Indians! He was a Congressman, a preacher, a historian – he did a lot of things, he had a whole life, short as it was. I actually visited his memorial, his grave, last year in Blackpool.”

Jackson with Alexander Skarsgard in The Legend of Tarzan. Photograph: Jonathan Olley/AP

You heard that right: Blackpool. Lancashire. Williams died in Blackpool in 1891, of tuberculosis, on his way back to the US after making an important intervention in the Belgian-backed proto-holocaust against the people of the Congo. He documented the cruelties of the Belgian rubber-harvesting industry there – maimings, executions, atrocities without number, millions dead – and on his return pointed fingers at both King Leopold of Belgium (to his face, no less) and his local agent, the explorer/exploiter Henry Stanley, implicating them in what was not yet termed a “genocide”. It was an important milestone on the long road to ending the horrors of the rubber trade.

Thus is the nightmare of colonial Congo grafted on to the fantasy universe of Edgar Rice Burroughs’ Tarzan series. The villain of the piece, played by Christoph Waltz, is another real-life figure, colonial administrator and mass killer Captain Léon Rom, likely one of the inspirations for Conrad’s Mr Kurtz in Heart of Darkness. This is something like the 200th Tarzan movie since 1918, but the first major reboot since the failure of Greystoke in 1984; The Legend of Tarzan seems prepared to situate itself amid some very dark and troubling history.

“Hopefully with this movie, we can persuade people to look into George Washington Williams’ story and, through him, find out about that first holocaust in the Congo,” says Jackson. “Williams is in the movie trying to convince Tarzan – who hasn’t been in Africa for 20 years – to go home and investigate King Leopold. He’s talked to some soldiers that were there doing bad things and he wants to find a way to stop these things. He wants to prevent the British and American governments from helping Leopold build his railroad. Because Leopold ran out of money – though how the fuck he ran out of money I don’t know, because he was pulling diamonds and rubber out of there, and rubber was like liquid gold at the time. Anyway, he ran out of money and that’s when he enlisted that army to go there. And that’s where we come in.”

Jackson as hitman Jules in Pulp Fiction (1994), the film that projected him into the mainstream. Photograph: Allstar/MIRAMAX

The Legend of Tarzan reunites Jackson with screenwriter Craig Brewer, who conjured up a magical role for Jackson in the steamy 2007 racial melodrama Black Snake Moan, and who is intent here, as in his movies as a writer-director (Moan, Hustle and Flow), on creating provocative black characters. This, of course, is meat and drink to the near-workaholic Jackson. When he read the script for Django Unchained, in which he was offered the part of Leonardo Dicaprio’s obsequious “house-negro” Stephen, the most painstaking evisceration of the Stepin-Fetchit-“yass-massa” caricature ever, and a role requiring immense delicacy and good judgment, Jackson said to Quentin Tarantino: “‘So you really want me to play the most hateful black character in cinematic history, huh? OK, let’s do it!’”

We often forget that before exploding into the public consciousness in 1994, aged 45, as Pulp Fiction’s fire-and-brimstone-spewing hitman Jules, that Jackson was a well-regarded New York stage actor. Well-regarded, that is, except for his demons and appetites. He played important roles in the first runs of a couple of August Wilson plays, but was always replaced before they moved to Broadway. He had started boozing, smoking weed and doing LSD at college in the late 60s, and has said that until he got clean in 1991 – after a crack-induced meltdown that involved his eight-year-old daughter finding him zonked out in the kitchen among his dimebags and paraphernalia – he had never set foot on stage without some kind of substance in his body.

“‘Made it’ is all relative,” he says of his supposedly late start in the movies. “I had a very good theatre reputation. Granted, I was a fucking drug addict and I was out of my mind a lot of the time, but I had a good reputation. Showed up on time, knew my lines, hit my marks. I just wasn’t making a lot of money, but I was very satisfied artistically. I was doing Pulitzer prize-winning plays. I was working with people who made me better, who challenged me. So I was doing things the right way, it was just that one thing that was in the way - my addiction. And once that was out of the way, it was – boom! The door blew wide open.”

As Stephen in Django Unchained. Photograph: Allstar/THE WEINSTEIN COMPANY

Getting clean freed up a hitherto closed-off dimension to his performances, he remembers. “I’ve always had my wife LaTanya, who’s my harshest critic. She’d say: ‘You’re so intelligent that the first time you read something, you think you understand it intellectually and emotionally, then you find the vocal inflections, and the facial expressions – and you can get there with that. But there’s no blood in it.’ And I’m like: ‘It’s all fuckin’ make-believe, what in the hell you talking about?’ And it wasn’t until I got sober that I knew fully what she meant. Before, I used to do stuff on stage and kinda look for the reaction from the audience – ‘Aha! I got ’em good that time!’ And once I was able to ignore that, and focus on the relationships with the people I was onstage with, I was finally able to blossom into whatever I might think I am now.”

Jackson started acting at Morehouse College, the famous post-civil war institution for sons of the emergent black middle class. But he felt stifled there by the generation gap between a cautious, gradualist college administration and the impatient young students who saw the civil rights struggle unfolding on TV every day. Jackson also had a dauntingly low number in the draft lottery. “I pretty much had to stay in school. Either I make good grades in school or I’m getting shipped off to Vietnam, which made it very real to me indeed when I was 19.

“Morehouse was breeding politically correct negroes. They were creating the next Martin Luther Kings. They didn’t say that because, really, they didn’t want you to be that active politically, and they were more proud of the fact that he was a preacher than that he was a civil-rights leader. That was their trip: they was into making docile negroes. We wanted a Black Studies Department, but they wouldn’t do it.”

With his wife LaTanya, 2016. Photograph: Barry King/Getty Images

But history had come home from the hill. “And all of a sudden things kinda went haywire on them. I met guys in my freshman class who had already been to Vietnam – they had afros already! Guys that had killed people in a war zone and knew what was goin’ on, and had discipline and leadership, those guys got hold of us. And suddenly we were talking politics and finding out how the war was getting run, who was getting killed. And what with civil rights going on at the same time, and with us being in the south, there was just no way you could ignore it.”

Jackson quit college for a year in 1968 to work with the Black Panthers – he was one of King’s many pallbearers at the funeral – until the FBI came to his mother’s house and told him he risked getting shot. He went back to school and graduated in 1971. That is a lot of history and politics for one 22-year-old, and shaped for ever the way Jackson frames and comprehends the world, and America.

I ask him how he rates the first black president, and what he thinks of Donald Trump’s inexorable rise.

“Obama got a lot done in seven years, hell yes! He’s done as much as he can do in the face of a morass of people trying to prevent him getting anything done. The fact that he got anything done at all is a miracle. He had to wait a minute until he could gangsta his way into doing all the things he wanted to do, because that was the only way it was gonna happen. He wasn’t gonna be able to do it all through the process. Those guys on the other side weren’t gonna go to work for him – they just stopped working, decided they’re not gonna do anything. And even though he’s created a lot of jobs, they’re not the kind of jobs that people used to have, and all the factory work is gone abroad. It’s created the environment that allows Donald Trump and that Rafael Cruz to thrive.”

Much as he despises Trump and his ilk, Jackson has only a limited degree of sympathy for those bedazzled by their race-baiting, scapegoating rhetoric.

With Ruby Dee in Jungle Fever, 1991. Photograph: Everett/REX/Shutterstock

“There’s a bunch of disenfranchised people whose anxieties they exploit. And those people are losing their jobs, and are going to be extinct. And they’ve convinced them that the cause of their extinction lies in immigration law. And they say, there’s a threat to your existence because these people are comin’ in stealing your jobs. People are getting back in the streets, but it feels like they’re asking for the wrong things. The only people who’ve figured out what they want, unfortunately, is Isis. And the rest of us are still trying to figure out why’re they doing this? That’s the crowd Trump knows he’s got, and he knows how to speak to them – and the truest thing he’s said all through the campaign, the only true thing, is that he ‘loves the uneducated’.”

In the 70s and 80s, when there was a thriving theatre world for black actors in New York, Jackson worked with everyone. “We worked all the time, and every now and then ... like, we were doing A Soldier’s Play and Denzel [Washington], he got plucked out to do St Elsewhere. Then Fish [Laurence Fishburne] got plucked out, and Howard Rollins, and then I was doing Mother Courage and all of a sudden Morgan Freeman was gone. He did Street Smart, and boom – he’s on his way. Every year somebody got plucked out. So I figured, I seem to be in the right place, so the right opportunity is gonna present itself in the end.

“Little by little by little, the landscape changed. I played that robber in Coming to America, which caught a lot of attention. But you always had to play the bad guy still, every time. You’d look at the script and go: ‘Let’s see, which page do I die on?’ Page 50? – aw yeah!”

After a half-decade of playing “gang member”, “hoodlum”, “robber” and Jackson’s own favourite (from Sea of Love) “black guy”, Spike Lee’s Jungle Fever broke him, well before Pulp Fiction made him a household name. His role as a crackhead son of a minister came literally weeks after his own rehab, and changed everything.

“I used to have this sort of ritual I had with my first agent. I’d call him up and always say: ‘Hollywood call today?’ ‘No!’ So after we went to Cannes with Jungle Fever, I called him up and said: ‘Hollywood call today?’ ‘Well, as a matter of fact, they did!’ But even then, that was, I think, my fourth movie with Spike.”

And once they had found him, film-makers and screenwriters couldn’t get enough of him. And Jackson, who replaced addiction with some vigorous strain of workaholism, has stayed busy for the entire two decades since.

But no Oscar yet?

“If I cared about that, I could have made a big fuss and gone all OscarsSoWhite and all that for Stephen in Django, but I don’t think an Oscar is gonna validate my career. You ask the average person, you know what they’re probably gonna say? ‘He already has one!’”

The Legend of Tarzan is out on 8 July.