The great Chiwetel Ejiofor is contemplating the inevitable business of allowing corners of your private life to become public. He doesn’t much like the idea.

“Well I never want anyone watching the work to be influenced by something they’ve read online or whatever,” he says. “I can tell you almost nothing about the private lives of all my favourite actors. It’s important to the work as a well as yourself to keep something back.”

Well, this isn't promising. Now 38, Ejiofor has not made much impression on the tabloids. He is never pictured falling in or out of Kim Kardashian's limousine. He has never publicly punched a policeman. A well-spoken man from a middle-class background, Ejiofor has kept the mask securely in place since securing his first significant role in Steven Spielberg's slavery drama Amistad 20 years ago.

Can he keep it up? The Oscar nomination for Steve McQueen's 12 Years a Slave nudged him to another level. That searing performance alerted those hitherto unaware of Ejifor that he was a contender for the best actor of his generation. People are now watching.

“Yes, that did have an impact on the industry,” he says. “It’s an amazing thing to have that broad impact. Oscar nominations matter. But the search is still the same. You’re still looking for great parts. That never changes.”

As it transpires, Ejiofor is actually pretty good chat. Set him on a subject and he will deliver an answer in neat, fluid sentences that fit together into well-structured paragraphs. You do, however, sense him rationing the release of personal information.

I wonder if he is prepared to discuss a tragic incident from his childhood. In 1988, when visiting Nigeria, he and his father were involved in a savage collision with a lorry. His dad, a doctor, was killed and Ejiofor was left with scars that are still visible today.I imagine he may not remember much of the incident itself.

“No, not very well,” he says patiently. “You don’t remember the event very well. But you do remember the aftermath. It’s a long time ago. I’m not an old man, but you end up having enough distance to rationalise the situation. You do get a sense of perspective.”

He ends his answer with a polite formality that suggests he’s done his duty. Following that catastrophe, his mother, Obiajulu, continued to work as a pharmacist and managed to put her bright son through Dulwich College in south London (alma mater of PG Wodehouse and Raymond Chandler).

“She was a resourceful woman,” he says. “She had to raise four kids on her own. The love and investment she put in were extraordinary.”

He began acting at school and, by the time he left, had already secured a few paying jobs. He doesn’t remember any fights about his decision to advance on the London Academy of Music and Dramatic Arts (Lamda). “There were a few things that allowed me to present it as a fait accompli,” he says. “That removed the argument about it. I had already been working. I had got a scholarship to Lamda. So I was able to present it as: ‘This is what I am going to do’, not: ‘Do you mind if I . . . ?’ ”

So there was less chance of him becoming a bum in the short term anyway. “That’s right. Ha ha ha.”

Full-on movie star

Spielberg plucked Ejiofor out of Lamda after just one year and propelled him into Amistad. Although he has only recently become a full-on movie star, it looks as if he didn't spend many of the early days living on spaghetti hoops while working in Burger King. As early as 1995, he played Othello in the Bloomsbury Theatre (a role he repeated, opposite Ewan McGregor's Iago, to even greater acclaim in 2007). He played Romeo and the young Peer Gynt. You can see him in Spike Lee's Inside Man and Stephen Frears's Dirty Pretty Things. This week, he appears as a conflicted hoodlum in John Hillcoat's terrific heist thriller Triple 9.

“Yes, I was always very lucky,” he says. “I always had a job and I was always able to turn down things I didn’t want to do. That allowed me a pathway into the working life. I was incredibly lucky to work with Spielberg so early. That opened a lot of doors. They would at least see me.”

I had read in an earlier interview that he was urged as a young man to change his name to something less Nigerian. The notion was that an actor called Chiwetel Ejiofor would only ever get cast in African roles. His reply to the journalist suggested that something a little like that did happen.

"Yes. But the intimation in that article was that it was a negative thing to play African parts, which I certainly don't think it was. I never felt that playing a Nigerian for Stephen Frears or another African for Steven Spielberg was a negative thing to be doing. I encourage African roles. I still have a deep connection with Africa and Nigeria in particular. I am desperate to tell stories from that part of the world."

There is no avoiding the issue of race this awards season. Two years ago, Chiwetel lost out to Matthew McConaughey at the Academy Awards, but Lupito Nyong'o, his costar in 12 Years a Slave, won best supporting actress and Steve McQueen was one of the recipients of a best picture gong for the same film. The Somalian Barkhad Abdi was also best supporting actor nominee for Captain Phillips. This year there is not a single person of colour among the acting nominees.

“Yes. I think it’s a very important to engage with this. It has to do with what kind of society we want,” he says. “We have to make sure people of all types are represented. The worst thing for any society is to have people who aren’t allowed to achieve their highest potential. It’s a tragedy if people feel that’s the case – if they believe there are barriers. That’s a terrible thing. We all have to make sure that this is not the case.”

Representations

He's goes on to point out that there are advances in how cinema represents race, gender and sexuality. "Look at films like Carol or Suffragette," he says. True enough and, though he's too humble to say it, consider Hillcoat's violent, thrilling, nasty Triple 9. Ejiofor plays one of several criminals who find themselves entangled with the Russo-Jewish mafia in Atlanta. In the days before Denzel, such a lead role would rarely have gone to a person of colour.

“Yeah definitely. And it’s important that we are investigating different sections of society.”

On a more trivial note, it's good to see Ejiofor getting down and dirty in a proper action film. He gets to wave guns, drive cars rapidly and face down Kate Winslet's terrifying crime kingpin. "It's a different way of extracting a character," he says. "I actually had to go through a physical change. When I first sat down with John Hillcoat I realised the character was a lot stronger than me. I was doing a lot of work with weapons and tactics. That was a new way of entering a guy psychologically."

I don’t imagine they train you in that stuff at Lamda. “No,” he chortles. “There were no classes in the use of the AR-15.”

It’s a gun, apparently.

Get the car: Five great heist movies

Rififi (1955) Blacklisted director Jules Dassin successfully offered two fingers to Hollywood when he travelled to France for this classic heist flick. Jean Servais is the veteran gangster planning an impossible jewellery robbery in Paris.

The Killing (1956) The film that properly pushed Stanley Kubrick above ground, The Killing stars Sterling Hayden as one of several undesirables plotting to rob a racetrack. Things go right. Then they don't.

The Italian Job (1969) There is plenty wrong with Peter Collinson's often chaotic film (right), but the concluding heist – in which mini motor cars jam Turin – is a masterpiece of absurdity.

Heat (1995) There is more to Michael Mann's thriller than that bank heist, but the smooth, thrilling sequence – scored to Force Marker by Brian Eno – is among the most influential in 1990s cinema. See Triple 9 for evidence.

The Thomas Crown Affair (1999) A controversial choice. What sort of idiot would prefer the Pierce Brosnan version to the Steve McQueen film? Our sort. The theft of the painting is slick. Its replacement is even better.