You may know David Oyelowo by his regal bearing: as Henry VI in the RSC’s 2001 production of Shakespeare’s trilogy; as Martin Luther King Jr in Selma; as Bechuanaland prince Seretse Khama in A United Kingdom, and even as himself, scion of the Oyelowos, a royal family in western Nigeria.

In his new film, however, the 41-year-old British actor is switching it up. “I was looking for something on the lighter side, having done a fair bit of dramatic work,” he says of Gringo, a south-of-the-border cartel caper. “I have a goofy side I realised that the audience hasn’t yet seen. That’s partly why I wanted to do it.”

Gringo tells the story of Harold Soyinka, a good-natured, downtrodden suburban cuckold, who is sent to Mexico as a patsy for his scamming corporate bosses, played with relish by Joel Edgerton and Charlize Theron. Oyelowo didn’t initially seem like a fit for the starring role. He explains: “He was written as Harold Salinger and he was definitely a white guy living in Chicago.” Thus, Oyelowo’s involvement began as a slightly awkward conversation with the director, Nash Edgerton, elder brother of Joel. “I sat down with him and asked: ‘Have you ever thought of a black person in this role?’ And he said: ‘Actually, I haven’t … I’m ashamed to admit that … It’s because I’m a white guy, so I just saw him as a white guy.’”

Oyelowo reports this evenly, having had versions of the same conversation many times in his career – neither his role in BBC espionage drama Spooks nor in the recent Cloverfield Paradox were “written as black”, he says. Nor was Emerson, his character in Jack Reacher, or the district attorney he played in the 2014 Oscar Isaac/Jessica Chastain crime drama A Most Violent Year. “Because, y’know, when I get out of bed in the morning, my first thought isn’t: ‘I’m black.’ I have concerns, I have hopes, you know, and that’s what you want to see in a character.”

With Oyelowo’s input, Gringo’s Harold became a first-generation Nigerian immigrant, whose inclusion elevates a simple crime comedy into a more interesting interplay of race, class and colonialism. “It is very evocative of my parents’ experience, being immigrants to the UK and being taken advantage of in ways they probably wouldn’t have been if they were in their comfort zone in Nigeria. When you’re outside your comfort zone, you are more dependent on other people doing what they say they’ll do.”

As a husband of 20 years, a father of four and a committed Christian, Oyelowo is also interested in the film’s ethical questions. “Certainly to be good, come what may, is something I like to think I aspire to, but I definitely know there are circumstances under which you can be taken advantage of.”

Where previously Oyelowo has played his morally upstanding characters as strong and noble, Harold’s goodness is an oft-exploited weakness. Yet this character still conforms to type: Oyelowo rarely plays villains, steers clear of the pimps, drug dealers and racially stereotyped “magical negroes”, and in 2015 he told an interviewer for NPR: “Don’t send me your script if you want me to play the black best friend. I just won’t do that.”

He has felt strongly about this for a while – Get Out’s Daniel Kaluuya recently reminded him of an exchange between the two actors in 2006. “He said [adopts accent of a north London teen]: ‘So, would you do EastEnders?’ And apparently I, very snobbishly – and I apologise to anyone in EastEnders or who likes EastEnders – went: ‘Nooooo!’ He said: ‘You know what? That never left me, you know.’ Whatever that means.”

This last equivocation sounds like undue modesty. What Kaluuya meant, presumably, is that Oyelowo’s demonstration of self-respect was hugely inspiring to a then-16-year-old boy, who was just starting out in the same field. “Yeah, I guess so. And I’m just so proud of what he’s achieved. But yes, I had a very specific set of parameters for myself.”

How does it feel to watch as this younger generation of black British actors achieves Hollywood success? Oyelowo’s reflections sound somewhat bittersweet: “I’m really, really proud, because they are now doing that which my generation wasn’t afforded. Y’know, the Leonardo DiCaprios and the Ryan Goslings, they get to break earlier than black actors do. You sort of need to pummel and plough away for longer, as a black actor, to get a degree of fame and, more often than not, you have to play a historical figure somewhere; basically a role that a white actor couldn’t play. But John [Boyega] is in his early 20s with Star Wars, Daniel is in his 20s with Get Out, and now Black Panther. Getting these opportunities younger means they have a greater length of time to build a body of significant work. If you look at myself, Idris [Elba], Chiwetel [Ejiofor], it’s happened for us a bit later.”

So although Oyelowo isn’t in Black Panther – superhero films are another personal no-no, for fear of typecasting – that hasn’t stopped him celebrating Ryan Coogler’s record-breaking Marvel movie. Oyelowo was on the red carpet for the Hollywood premiere, wearing a dashiki, and he speaks passionately about the significance of the film, particularly the representation of black women.

“I know those warrior women, they’re my aunties, they’re my cousins, they are my mother; we just don’t get to see them on screen. Enough with the mammies, enough with the slave women. Enough. We’ve seen that, we know it’s a part of our heritage, but so is this.”

He’s also eagerly anticipating the revival of the long-mooted adaptation of Chimamanda Ngozi Adichie’s Americanah, to which he is attached, along with two of Black Panther’s female stars, Danai Gurira and Lupita Nyong’o. “I just love that book so much,” he says.

Actively supporting the careers of women in film is another good habit Oyelowo has been cultivating. In his capacity as producer-star, he brought Amma Asante on board to direct A United Kingdom, worked with Mira Nair on Queen of Katwe and with Selma director Ava DuVernay. “I guess, to a certain degree, it’s pragmatic because those are working environments I enjoy,” he says. But it’s more than that, too: “I find it particularly egregious that talented people anywhere are being marginalised … the idea that women, and women of colour in particular, are getting few opportunities, that just doesn’t make sense to me because they have such a rich contribution to make. It’s just … I’m indignant about it. I find it incredibly annoying.”

Most actors don’t have a plan for their careers – or won’t admit to one. Oyelowo not only has a plan, but a conception of his place in an arc of black cinema history that stretches back to Sidney Poitier and forward, to an upcoming generation. “There is a degree of activism in terms of the roles I take.” He says he is aware that his work has resonance for younger black actors, just as, growing up, he absorbed the work of older actors. “They suggested to me what was possible, they gave me a sense of who I could or should be in the world, for better or worse.”

So, if you imagined this flexing of comedy muscle in Gringo was a sign that Oyelowo has relaxed his career-guiding principles, you would be wrong. “I’m still at a place where representation in movies is too important to relax quite yet. Maybe I’ll play the best friend when I’m 80, but until then, there’s work to be done.”

Gringo is released in cinemas on 9 March.