Hollywood actress Thandie Newton would be the first to admit that she used to have a very low opinion of British TV. “If aliens landed and saw all the crap on TV, they would be shocked,” she says. “As the comedian Stewart Lee puts it, ‘TV is warm diarrhoea that we’re being spoon-fed.’”

But Newton’s opinion changed radically the moment she was offered the guest lead role in the fourth series of Line of Duty, Jed Mercurio’s compelling police corruption drama. “I don’t generally watch TV,” she continues, “but my husband and I sat down and watched the third series of Line of Duty in one go. I was so thrilled by it. The fact that Jed dared to dispose of the star, Daniel Mays, in the first episode – I thought that was genius.

“It seemed to me like a return to the golden age of TV. I get goose-bumps now thinking about shows I grew up watching like The Singing Detective. What a privilege it was to be alive at that time. And now we have Line of Duty. So even before reading the script, I was in. I really wanted to be involved in the best thing on British television. So I met Jed the very next day and said to him, ‘Whatever you’ve got for me, I want to do it!’”

The regular stars of the show were equally delighted that Newton agreed to guest in the fourth series of Line of Duty, which makes its BBC1 debut after after three series on BBC2. Award-winning actress Vicky McClure, who plays anti-corruption officer DS Kate Fleming, confesses that at first she couldn’t believe they had secured a bona fide Hollywood star of Newton’s magnitude. “I was like, ‘Thandie Newton?’ I couldn’t get my head around it. But we had so much fun together. She is an extremely bright and talented woman who has tremendous presence. Now I call her ‘Hollywood’!”

Newton (left) joins 'Line of Duty' regulars including Vicky McClure who plays Detective Sergeant Kate Fleming (right) (BBC)

Newton plays DCI Roz Huntley. A high-flying detective who has returned to the job after taking some time out to bring up her children, she is heading up the career-defining Operation Trapdoor.

Under enormous pressure to solve the case and prove herself to her superiors, Huntley apprehends a suspected serial killer. But after suspicions are raised by Huntley’s meticulous Forensic Coordinator Tim Ifield (Jason Watkins, W1A) about the evidence collected in the case, it appears that a miscarriage of justice may have occurred.

So Huntley becomes the focus of an investigation by the relentlessly beady-eyed officers of the police anti-corruption unit, AC-12: Fleming, DS Steve Arnott (Martin Compston, In Plain Sight), and Superintendent Ted Hastings (Adrian Dunbar, Ashes to Ashes). As it considers the question of who is policing the police, Line of Duty once again proves itself to be an irresistible watch.

As part of the AC-12 investigation, Huntley is the subject of one of this series’ now-trademark extended interrogations. But the detective under suspicion turns out to be more than a match for the investigating officers.

Thandie Newton and Matt Dillon in Crash (c.Lions Gate/Everett/REX Shutterstock)

Huntley’s calmness under fire was one of the elements that drew Newton to the role. “Does she lose her cool during those interviews? Not really. She’s had to fight to get where she is today, and she copes very well with this trial by fire. I love the fact that we’re seeing a woman in that position with hide that strong. She is bullet-proof, at least metaphorically.”

For all that, the 44-year-old actress, who won a BAFTA for her performance in the 2004 Oscar-winning movie Crash, acknowledges that shooting those scenes was very demanding. “I had to take a deep breath beforehand. I’m a perfectionist, and so if I get one word wrong, I give myself a hard time. One 24-page interview scene really did me in.

“But in a way that was good for the characters,” adds the actress, who has starred in major Hollywood movies such as The Pursuit of Happyness, W., and Mission: Impossible II. “The more unravelled we get in those scenes, the closer the facade comes to cracking. At the end of a 14-hour day, you’re close to breaking point. As a character, you’re desperately trying to keep it together.”

One of the many strengths of Mercurio’s scripts is that they keep you guessing. The viewer is never quite certain where she or he stands. Jason Watkins, 50, who picked up a BAFTA for his performance in the title role of The Lost Honour Of Christopher Jefferies, observes that, “The great thing about Jed’s writing is that not only does the plot keep twisting and turning, but also you are never quite sure where you are with the characters.

”I play someone who seems to be a grumpy, middle-aged, technical guy. Then he does something unexpected and you think, ‘Hang on, why is he doing that?’ When you’re watching Line of Duty, you’re constantly confounded. And that’s why it’s such a joy.”

The actress stars as Maeve in 'Westworld'

In the same way, Huntley is certainly not depicted as an unambiguous villain. Newton, whose mother is Zimbabwean and whose father is English, contends that her character in Line of Duty should be admired in some ways, as she has had to overcome all manner of prejudice to reach her current exalted position. “Every woman from every line of work will recognise the frustration here.”

The actress, who lives in London with her husband, screenwriter Ol Parker, and three children, continues that, “If you’re a woman, you have to be twice as good, and if you’re black, you have to be twice as good on top of that. The audience will see that. This show will force them to be judge and jury.”

This series of Line of Duty also highlights a very topical subject, the idea of “alternative facts.” Some of the comments flying around the US political scene at the moment have emphasised that we really do seem to be living in a “post-truth” world.

Mercurio, 50, a qualified doctor, reflects that, “There is a lack of respect for facts these days – anyone who’s been to a dinner party and listened to someone talk about homoeopathy will know what I mean.

“Where is objective reality and how do you test it? Criminal justice is a good way of exploring that. I’ve become more and more exasperated by the lack of respect for facts and people’s inability to tell the difference between fact and opinion. That is something that, unfortunately for the world, has become a big issue.”

Newton’s magnetic performance as Huntley should remind British TV commissioning editors that they have a very considerable talent on their doorstep. The actress, who scooped a Critics’ Choice Award for her performance as sentient android Maeve Millay in sci-fi western mind-bender Westworld, asserts that she would love to work more in this country. However, she is not holding her breath.

Newton in the film 'Mission Impossible II'

The actress, whose career continues to thrive in the US and who has a part in the as-yet-untitled new Han Solo Star Wars movie, sighs that the paucity of parts for black actors on British TV “Is a real shame, but it’s people like me who can change it”.

She continues: “Rather than moping around or getting really stressed out, I’m trying to do things to make that difference. I want to be on British TV, so I’ve written a six-part series, which I hope to make next year.” With a laugh, she adds: “Give me glass ceilings wherever I go, and I’ll just smash through them!”

The other thing that is holding back Newton’s UK TV career back is that “It’s all costume drama in this country, darling”.

“They’re not going to put a person with brown skin in any of those”, she says. “That’s the fly in my appointment. I’d love to go to my own bed every night and see my kids, who go to school in this country. I’d love that, but it’s never been possible. I’ve been in 40 movies and only four have been shot in the UK. That’s a disgusting statistic.

“If I could be in Downton Abbey or Victoria, what would I be? A maid? No, thanks!”