Kerry Washington is doing it for her father.

The 35-year-old New Yorker takes on the role of an enslaved wife in Quentin Tarantino's Oscar-nominated Django Unchained – a violent Western that erupts amid the world of slavery, setting Jamie Foxx's titular black hero on a mission to gun down his white oppressors as he bids to rescue his spouse.

"I am making this film for my father," begins Washington, "because my dad grew up in a world where there were no black superheroes. There were people who were working to break barriers but they weren't people in pop culture. For him it was, 'I am a young African boy and there's nobody who looks like me who is changing the world.' So to be part of this epic story is important to me."

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For all its personal resonance the role came at a price – Django Unchained, nominated for best film in the Oscars, is by far the most ferocious piece of cinema in which Washington has starred and she says she found the process disturbing. Her character is called Broomhilda, a name that recalls the ancient Germanic legend of Siegfried, in which the hero battles all-comers in a bid to win his one true love.

In Tarantino's world, Django wears Siegfried's mantle and he sends many a soul to hell. The film is the Reservoir Dogs and Pulp Fiction creator's most vicious offering – the Kill Bill movies revelled in aesthetic violence, but this is a realm much more sinister and cruel. Some on the American right have raised a stink about the slaughter of whites by the black cowboy, but their complaints are ill-advised; the violence that truly shocks is that meted out by the white slave-owners.

Tarantino, for example, plumbs his knowledge of cinema to revisit "mandingo fighting", where slaves must fight to the death with their bare hands (the 1975 film Mandingo, drawn from a 1957 novel of the same name, used the same ferocious idea). Later, another horrific scene – carefully edited to minimise audience trauma – sees wild dogs let loose upon a runaway mandingo fighter and the brutality of that world is laid bare. "It disturbs me every day," Washington tells me, while still shooting the movie, "but I also think in the United States and all over the world we don't deal with the real violence that was the institution of slavery."

This year will see the release of Steve McQueen's Twelve Years A Slave, starring Chiwetel Ejiofor, while Steven Spielberg's Lincoln opens on Friday, and Washington believes it is important to remind people of the horrifying truth of that world.

"Nobody really depicts it because when you read about it, it is unfathomable to think that people were treated this way," she says. "Even if the film has nothing to do with my personal, modern experience, it feels personal because of where I come from.

"I would not exist if there were not people who had been strong enough to live through these experiences. So there is the incredible sense of gravity, a responsibility that you have within the context of this fantastically entertaining story, in terms of showing honour to the humanity and dignity of these people.

"It's almost as if Quentin is the only director who would be willing to go into that dark place, because it was so awful in reality that nobody has even come close to showing what it was like. And it's great that it's Quentin – this is not a documentary. It's a Tarantino movie which happens to be within this important historical context."

For all Washington's passion for – and importance to – the story, Django Unchained remains Foxx's tale, Django taking on a heroic quest in which he is accompanied by his friend, guide and emancipator, a dentist cum bounty hunter played by Christoph Waltz.

The latter scooped an Oscar for his performance in Tarantino's last movie, Inglourious Basterds, which had a pronounced Western feel to its opening chapter, and he's already earned an Oscar and a Golden Globe nomination for Django Unchained.

As it transpires, however, Waltz says he has no real passion for the Western as a genre – "I kept trying to get into Once Upon A Time In The West when I was 12 and it was playing in Vienna," he recalls, "but I failed" – and yet he appreciates what this particular world offers to filmmakers and audiences.

"I think people like the Western because it is extreme drama about life and death and usually about truth and betrayal," says the actor. "It's about the basic fight for survival. In a Western you expect life to be at its furthest extreme. The bad guy is not just bad. He is really bad."

One of the bad guys in Django Unchained is a Southern plantation owner brought to life by Don Johnson (enjoying something of a John Travolta rejuvenation moment). He isn't the really bad guy, though. That dubious honour falls jointly upon another plantation owner, Broomhilda's keeper – played by Leonardo DiCaprio, who tackles the part with an arch malevolence – and his black major domo, brought to the screen by Tarantino favourite Samuel L Jackson.

"The past few movies Leo did were very serious, and very involved, and very dramatic, and he is such a fabulously charming person," says Waltz of DiCaprio's involvement as the film's crude, aristocratic villain. "I always thought I would love to see him do a part where he can actually employ this beautiful openness, charm and this versatile and graceful style." He smiles. "But then this part comes along."

DiCaprio proves a suitably powerful and menacing adversary for Django, who once freed from slavers by Waltz goes head-to-head with the plantation owner in a hail of blood and bullets. Like Washington, Foxx believes that beneath the action and excitement an important story is told. "I think this is a groundbreaking film," says Foxx. "You haven't seen anything like this."

When Tarantino first began casting for his film, superstar Will Smith was linked with the starring role, though when other commitments forced him elsewhere, Smith lobbied for Foxx's inclusion as he had done on Ali many years before.

"It's a dream and I owe a lot to Will," says Foxx. "As a kid, you twirl your little plastic guns or you ride your horses. I'm from Texas and being from Texas, when you grow up, everybody did the same thing – we all watched Bonanza. And then, the next thing you know, something like this comes along.

"I always say great projects parallel your life, and about five years ago I got a horse for my birthday and started riding. And the next thing you know I run into this cinematic genius called Quentin Tarantino. He says he has a Western and I say, 'Well, I happen to have my own horse.'" Foxx pauses for effect. "'I don't know who else could come in and tell you that.'"

Foxx's bid for the role paid off. "I met six guys, six magnificent actors, but Jamie was the cowboy," Tarantino says before pointing out that Foxx is the first actor since Roy Rogers to use his own horse in a Hollywood film.

Foxx (and his horse) travelled to a variety of locations with Tarantino, filming in California at Mammoth Lakes and at Santa Clarita, taking in the Melody Ranch, which has hosted a number of Westerns including the critically adored yet ill-fated Deadwood television series. The filmmakers also headed to bayous and plantations in Louisiana and the snow-capped peaks of Wyoming.

"There are actually more open spaces in this movie than I have ever done before," says Tarantino, "although I still have my chamber pieces, like dinner table scenes. And actually I feel the beauty shots are a trap Westerns fell into somewhere around the late seventies and into the eighties, and pretty much for Westerns ever since.

"They get caught up with pretty, poetic visuals and they go out there and they shoot these beautiful sunsets and they shoot these mountain ranges. Now, I'm not saying I haven't shot a sunset and I'm not saying I haven't shot a mountain range, but because a lot of filmmakers get caught up in the exquisiteness of the story that's why a lot of people in the last few generations think Westerns are boring.

"It's not just about the cloud passing over the horses as the sun breaks out. For us it is, 'We got s*** to do, we got s*** to talk about, we got people to kill.' So this movie is very much a mud and blood Western."

While the likes of Pulp Fiction, the two Kill Bill films and Inglourious Basterds all in part resound with a Western tone, Django Unchained is Tarantino's first full-scale foray into one of his favourite genres, and his spaghetti Western is flavoured with a typically broad range of ingredients. There's a new song by the genre's master composer Ennio Morricone which plays during a trademark Tarantino conversation scene, and plenty of moments to remind audiences of Sergio Leone.

It is the films of Sergio Corbucci, however, the Italian filmmaker whose fierce and flyblown Westerns from the sixties and seventies carry strong anti-fascist sentiments, that have proved Tarantino's greatest inspiration and whose influence gives Django Unchained its added kick.

Corbucci's West is extremely brutal and unforgiving and Tarantino tips his hat to the Italian not only with his film's tone but also its title, which recalls Corbucci's 1966 film Django and the numerous sequels and knock-offs it spawned (there's also a cameo from Django star Franco Nero).

"Sergio Corbucci to me is one of the greatest action filmmakers who ever lived," says Tarantino, "and he is definitely one of the greatest directors of Westerns. It was the themes he explored and the landscape he created. There is no Western landscape to match the Corbucci one when it comes to brutality, when it comes to violence and to the pitiless nature of the characters.

"That's what I love about the West that he created. That's an interesting landscape to tell a story in. You think, 'How would you come up with a Corbucci landscape today which has that much brutality and surrealism, and such a pitiless nature?' A black man in the South before the Civil War, that's a good start."

Django Unchained is the latest in a long if rather slender line of black Western heroes who popped up in the low-budget horse operas of the thirties and forties before becoming more common in the 1970s where the likes of Sidney Poitier filmed Buck And The Preacher, The Legend Of Nigger Charley and Duel At Diablo.

This Django, however, is among the first black gunslingers to confront the issue of slavery head-on. "I hope other people will realise what a fertile ground this is for stories," Tarantino says. "Not for people to wring your hands about slavery and to spread out the misery of what happened, but to find rich stories of heroism, of cowardice, betrayal, love, hate.

"People say there are no new stories out there, but there is a wealth of stories that can be told from different perspectives, that America has just been afraid to tell, whether they're white stories or black."

Tarantino has never lacked courage or conviction in his filmmaking although he concedes that the eight-month shooting schedule for Django Unchained proved testing in the extreme.

"On my movies we always have a good time because I believe it's a joy and a privilege to be able to do what we do," explains the writer-director. "My crews are my family and this is my time to live life to the fullest. But with this subject matter though, definitely, it's been a little surreal.

"We had a large black crew and we have a large black cast, all dealing with this subject, and everyone is dealing with it in their own way. And dealing with it in when working in the South, it brings up a lot of stuff. But then it's made us all very close. It was a moving experience."

Kerry Washington, for one, agrees. The actor recalls in more detail a vivid scene where she's whipped against a tree. "There were lots of trees around, all of them had moss on them," she remembers. "I said, 'How come that tree doesn't have moss?' And they said it's because it was a hanging tree, where the mosses weren't able to grow. That was quite chilling.

"And I remember a day when Jamie and I were shooting being branded with a hot iron. But that evening I was shooting an even more violent scene, so being branded seemed OK in comparison." She smiles. "It's been difficult. Making this movie was very intense." n

Django Unchained (18) is in cinemas now.