It says a lot about British actor Archie Panjabi that the best piece of advice she says she ever received was when an Indian agent told her that an Indian woman would never make it in Hollywood. It’s a story that would leave most of us shaking our heads at the sheer state of things. But for Panjabi, a gauntlet was thrown down. “It made me realise how hard it would be,” she recalls. A decade on from that advice, she is in London to promote her latest star-studded show, Departure, in which her ethnicity is as irrelevant as her stellar Hollywood status is germane.

Panjabi’s fans, of course, won’t be surprised. The actor has a 25-year-long, Atlantic-spanning film and TV career under her belt, as well as a reputation for philanthropic clout. She guest-edited last year’s special race issue of the UK National Geographic; she’s been consistently vocal about women’s rights, working on everything from Amnesty International’s Stop Violence Against Women campaign to the 10x10 project to promote education for girls. While she is more effusive and gentle in person than most of the roles she’s famous for would suggest (at one point her PR sneezes during our interview; she’s instantly asking if he’s ok, offering him a drink, handing out throat lozenges …), you sense she’s as tough as any of the characters she’s portrayed.

Of course some characters stay with you more than others. For many people, Panjabi will always be Kalinda Sharma, the scene-stealing private investigator she played on The Good Wife. Tight-lipped, fiercely loyal and utterly ruthless in her red leather biker gear, Kalinda trampled all over a multitude of stereotypes (racial, female, professional) to become one of TV’s most enigmatic and memorable figures.

It’s odd, then, that not 12 minutes into Departure, Panjabi appears on screen as … a tight-lipped private investigator in red leather biker gear. “It’s nice to know it was spotted,” says Panjabi when I mention this nod to her former character. But how could it have been missed? Daniel Lawson, the costume designer on The Good Wife, once tweeted his department had “approximately 60” jackets for Kalinda. The throwback here gave Panjabi a way of injecting some personality – “a bit of a rock chick” – into what could have been just another stern-faced security operative at work.

‘You’re the only one that can find this plane!’ ... Archie Panjabi with Christopher Plummer in Departure. Photograph: NBC

A gripping six-parter with a timely premise, Departure opens with a passenger plane going missing over the north Atlantic. Panjabi’s character, recently widowed aviation investigator Kendra Malley, is tasked by her boss, head of the Transit Safety and Investigations Bureau Howard Lawson (Christopher Plummer) with finding it. She rallies her troops – some of whom resent her being the first woman to head an investigation – and faces off with Howard, her mentor, when his loyalties seem divided between the truth and corporate games.

Despite some clunky dialogue (“You’re the only one that can find this plane!”), Departure proves a great whodunnit, expertly helmed by Orphan Black director TJ Scott.A Canadian-British co-production, the show is set mostly in London but is decidedly transatlantic in tone. Britain’s post-EU place in the world is intriguingly imagined. The airline under investigation is not BA, but British Global Air, while the aircraft is a product of the fictional Windsor Aeronautics. The conspiracy-theory-style plots take in Saudi and Chinese money, Russian meddling and Mossad impunity, with lots of gleaming metal and crisp power suits.

There are moments of welcome local flavour, too, some of which Panjabi threw in herself. “When I messed up a line, I said, ‘Bollocks.’ And the writers kept it in as a character thing.” The gruffly affectionate “Come on, soldier” with which Plummer addresses her wasn’t scripted either, and it instantly suggests the kind of bond only forged through long hours of hard slog.

Keep your friends close ... Panjabi with Julianna Margulies in The Good Wife. Photograph: Collection Christophel/Alamy

As Kendra’s team pieces together the story of the crash, her painful backstory rears up. Panjabi relished grappling with such internalised trauma. As an actor, she is the master of doing an awful lot with very little – silence, stillness, the subtlest facial movements. “I never like a lot of dialogue,” she says. Where some actors explicitly stipulate ‘no closeups’ in their contracts, she is a fan. She likens them to a secret language, an umbilical cord between her and her audience, a way for them to get inside her head.

She’s also a fan of what one might call ‘fromances’ between female characters: platonic chemistry, so often lauded between male colleagues on screen, is rarely afforded to women. There isn’t much of the latter in Departure, but she thinks the relationship between Kalinda and Alicia Florrick, played by Julianna Margulies, was central to The Good Wife’s appeal. The women weren’t pitted against each other, or “portrayed as bitchy or nagging”. It might also explain the enduring interest in that relationship – on screen and off – despite the show’s demise.

For years, beady-eyed fans have pointed out that the two actors didn’t share a scene for two and a half seasons (over 40 episodes) and that when they finally did seem to, for a farewell scene in a Chicago bar, it was an inexplicable split-screen mashup. Panjabi, who left the show in its penultimate season, won’t be drawn on what happened – and whether the persistent rumours of a feud are true. “Well, I think, my God, it’s so many years ago now,” she says, adding that she has addressed it before. Which is as much as Margulies has ever said too.

When she does expand on the topic she is evasive and careful with her words. “We had created so much intrigue around Kalinda, around my relationship with Alicia, I felt it was the right time, after six years, to go. And there still is a level of intrigue. Every journalist asks me about it now. And it amuses me. Because I feel like, yeah, she’s, you know, she’s still very much present.”

Panjabi was born in north London. When she was growing up, she says, “there was nobody who looked like me on TV”. Her parents’ qualms about her chosen vocation were always, ‘how can you have a career where there is nobody to look up to?’ So when EastEnders introduced a regular Asian character, shop-owner Naima Jeffery, it gave her, and them, a ray of hope. “I could be the next Indian girl.”

She was in her early teens when she was called – three times – to audition for the soap and remembers being heartbroken when she was rejected (for not having a union card). However, the UK was just a stepping stone: it was the US that properly opened the door for her. She got to do a lot more film, working with everyone from Fernando Meirelles (on The Constant Gardener) to Angelina Jolie on Michael Winterbottom’s A Mighty Heart. Not getting on EastEnders “didn’t turn out too badly.”

‘We’ve got to keep pushing’ ... Panjabi on diversity. Photograph: Linda Nylind/The Guardian

Panjabi embarked on that National Geographic race issue last year just as she was promoting ITV’s Next of Kin. It was her first lead role in a British TV show and the chance to portray what she described in the magazine’s intro as “a solid, interracial marriage”. By 2044, she wrote, non-Hispanic whites will make up less than half of the US population. “What would media that truly reflect our diverse world look like?”

She hasn’t always been this vocal about the need for change. She worked incredibly hard, she says, to get recognition for being an actor, not an Asian actor. But she relishes how #MeToo is now creating “a social conscience”. When people embark on a project these days, she says, they know they have to be diverse, otherwise they’ll be criticised. Panjabi’s outlook, though, is resolutely upbeat, idealistic even. Diversity, for her, isn’t just about black or white: “It’s about people with ginger hair. It’s about people who have speech impediments.” It’s about getting to a point, she says, “where there is no such word as ‘diverse’ on TV, [where] we don’t talk about differences.”

Stormzy’s Pyramid stage headline show at Glastonbury last month, where the British rapper highlighted the diverse dance initiative Ballet Black, felt like a big moment in that regard, I say, especially for a new generation. She concurs, with the kind of steady, steely gaze that says she is in this for the long haul. “I know people say we need to have more, but it takes time. Sometimes we need to stand back and see these changes are actually occurring right now. We’ve got to be grateful for them. And we’ve got to keep pushing.”

Departure airs Wednesdays at 9pm on Universal TV.