The actress on dressing down for the gruelling Line of Duty and motherhood and her marriage to Matthew Macfadyen

Keeley Hawes is officially a great English beauty. I know this because I have seen her in Tipping the Velvet, Spooks, Upstairs Downstairs and Ashes to Ashes. Because she has been the face of a Boots No7 campaign. And because, on the way to meet her in Richmond on a sunny spring afternoon, I happen across a tabloid article that devotes some 300 words to what she is wearing as she leaves a TV studio.

However, as we take seats in a chain restaurant I’m not interested in extracting beauty tips. I want to know the exact opposite: how she manages to look so terrible playing Detective Inspector Lindsay Denton in the addictive second series of the BBC Two cop drama Line of Duty. Did she put on weight? “Well, I didn’t keep it off,” she says, removing her sunglasses. And what about the dark rings under her eyes? “Sometimes that was make-up, sometimes it was just not taking off the make-up from the day before.” And what about the hair, which has been referred to as a “psychopathic fringe”. “It was my own!” She laughs and runs a hand through a hairdo that has been dyed red for another role. “It’s just a fringe. But we tried to make it look like she cut it herself.”

The transformation was so successful that during dubbing she heard sound editors comparing her performance to Charlize Theron’s Oscar-winning one in Monster — a backhanded compliment given that Theron gained 30lb for the role and wore prosthetic teeth. Though if Hawes found it amusing, it’s because the six-part series, the fifth episode of which is broadcast tonight, is the drama sensation of the year. It follows the police investigation into a botched operation that has left two police officers and a key witness dead. Hawes’s Denton is suspected of being involved in organising the killings and is either a cold calculating murderer or the victim of one of the most serious miscarriages of justice ever seen as, week by week, languishing in her prison cell, she is subjected to a new series of horrors after being refused bail. With just two episodes left, it’s still not clear if she is innocent of all charges or as guilty as hell, and the suspense is gripping.

Typically for something written by Jed Mercurio (Cardiac Arrest, Bodies), it is magnificently well observed — the extended beeps at the beginning of recorded police interviews, the chilling prison scenes where inmates are only allowed to respond with “yes” or “no” and the characters, none of whom is entirely sympathetic, make it all feel very real. But Hawes’s performance as the depressed and occasionally violent Denton, for which Clive James has said she should win “every award going”, is what makes it sizzle.

“I would have laid down on the road to get the part,” says Hawes, after ordering coffee. “It’s very difficult in our industry to break people’s idea of what you are. People have seen me looking as best as I can with the help of a team of people, so it is interesting to go in the opposite direction. It’s been the best reaction.”

Not that much of this response has come to her in person: she says the way she looks on screen means that “people have recognised me on the streets less than for anything I’ve ever done”. But the reviews have been so good that she has broken the habit of avoiding them, and although she does not watch the show as it is broadcast she has started glancing at reaction online. “I came off Twitter a few years ago, when someone asked me what I smelled like.” A wry smile. “But I came back because I wanted to tell people I was no longer in Barking in Essex [the play she left suddenly before Christmas after reports of rows with co-star Sheila Hancock, with one source saying that Hancock, a theatre veteran, had insisted that Hawes project her voice more]. No one else was putting forward anything official.” She returns to the subject. “The response has just been quite moving.” At least as good as The Killing and Borgen, I say. “Actually that’s something people keep saying — is it going to Scandinavia? It’s like people want to stick it back to them.”

The coffee arrives and while listening to an actor talk about their inspirations is normally the dullest thing in the world, I find myself asking Hawes if her performance was influenced by anyone. Some viewers have detected a little of The Bridge’s socially awkward investigator Saga Noren, though I detected everyone from Tori Amos (when Denton is banging away forlornly at a piano) to Susan Boyle. “Everybody probably knows somebody like Denton — someone who hasn’t got the job they want, whose relationships haven’t worked out, who is disappointed.” I joke that it sounds like me. “Exactly!” She laughs a little too hard. “I got a real sense of her loneliness in the very first scene I filmed; when she was heating up some macaroni cheese in a microwave and feeding her cat in her dingy flat.” She adds: “It was a lovely flat though. A lovely couple lived in that flat.”

This intense keenness not to offend, or as she puts it, “not to sound like a t***”, is a theme of our conversation. Hawes is not bland. She swears like a trucker. Manages a rant about the aforementioned tabloid article and the paparazzi shots used to illustrate it. And when I ask about Robert Lindsay, who was originally cast in Line of Duty but left part way through production due to “creative differences”, she says she was glad it happened before she started filming, adding: “I had enough s*** to deal with that play. It was miserable.”

When I mention all the good reviews she has been getting she feels the need to point out:“I’ve had some terrible reviews in my life.” When I ask what she makes of BBC Three being closed, she says: “I feel fond of it but if they are saving money then maybe there is a point.” About the dearth of good roles for older actresses, she just says: “I can’t complain.” And when I tell her she is relentlessly reasonable she worries about this too. “Am I? Oh God.”

She returns to the subject of female roles later on. “Look, when I am 45 and have no work, I might take this back, but I did my first work for the BBC when was I 9, riding a bike around Albert Square. I got a cheque for £19 and it has gone pretty well since. If I had been out of work for three years I could complain.”

I tell her that the last actress I interviewed, Romola Garai,was pretty cross about the clichéd nature of the roles that came her way. “She is quite militant isn’t she?” The inevitable correction: “And good on her.” She finishes her coffee. The thing is I’ve been married to an actor for a long time. [Matthew Macfadyen, whom she met on Spooks 12 years ago]. I see a lot of what comes through the door for him and it is not dissimilar. People just want you to recreate the same thing again.”

Or to put it another way: Hawes is as happy as DI Denton is miserable. The daughter of a London taxi driver, she attended the Sylvia Young Theatre School. Today, at the age of 38 and a mother of three, she is successful without being too famous (“If I am out with my husband, people sometimes do a double take, but mostly people just think they know me from school”). She and Macfadyen, who recently appeared in Ripper Street, have both recently had successful runs on TV (“a good couple of weeks in our household”.) And she is working in her preferred medium (“I read a review the other day that compared House of Cards with Line of Duty, which is great, right?”).

She glances at her phone and it’s time for the school run across the river in Twickenham where she and Macfadyen live. Macfadyen is currently starring in the West End comedy Perfect Nonsense and Hawes has to take on the parenting “baton” in the early evenings. As she gets ready to leave I can’t help asking if Lindsay Denton will be OK. She has had a rather bad time of it, having her head shoved down a loo by colleagues, excrement put in her porridge by a screw and the last we saw of her she was being bundled into the trunk of a car, while in tonight’s episode she is waterboarded with a wet towel.

“Actually, the script initially said that they would be urinating on the towel in question. But it was deemed too extreme.” Poor Lindsay. Will it end badly for her? “I can’t say.” I try putting the question another way: will Hawes be starring in the next series? “No. But the lead didn’t return with the previous series.” I say it’s a shame and she replies that she is now filming something linked to Line of Duty. Something written by Mercurio? “No.” A detective drama? “No . . . I can’t say, but it will be announced at the end of the series.” And with that she is off, protected against the world, should any celebrity journalists be reading, in a Topshop pullover, aviator shades and an Yves Salomon rabbit-fur-lined coat.

Line of Duty, tonight, BBC Two, 9pm