In National Treasure, the actor plays a veteran comedian caught up in a sex abuse scandal. On set with Julie Walters and Andrea Riseborough, he talks about brutal rehearsals and being moved to tears ‘without having to act at all’

The accused sits in the dock, a brooding black mass hunched stark against the arched windows of Bradford’s Victorian courtroom as the morning sun streams in. Even in silhouette, his profile is unmistakable: those prizefighter shoulders, that uppercut of a chin – who else could it be but Robbie Coltrane?

We are on the set of National Treasure, Channel 4’s four-part drama inspired by Operation Yewtree, the police investigation into the sexual abuse perpetrated by Jimmy Savile and allegations against other media personalities. Coltrane, now 66, plays Paul Finchley, a beloved veteran comedian suspected of historical sex offences. Between takes, the actor puffs on a vape, fumes swirling in the sunbeams, giving him the look of a tired old dragon at bay.

The director Marc Munden puts an arm around the actor and leans in close to murmur in his ear before announcing another take. In the witness box, Kate Hardie, playing a woman testifying against Finchley, accuses him of raping her more than 20 years ago.

Throughout this long scene, the camera remains on Coltrane. As Hardie’s testimony comes to an end, tears roll down his cheeks. It is an extraordinary moment. All these people on set, all the lights and lenses and fakeness, and yet at the centre this stillness and emotional authenticity – the only sounds, his breathing and her sobbing. Here is an overdue reminder of why Coltrane won all those Baftas in the 1990s for his portrayal of the criminal psychologist Fitz in Cracker. There is a fragility about this performance, though, which we have not seen from him before; Cracker cracked.

“I didn’t have to act at all,” Coltrane says later, referring to his tears. “The evidence was so heartbreaking. I mean, if you’re not moved by that, you might as well hand in your human-being cards.”

Is Finchley guilty or innocent? It isn’t clear. Ambiguity is a watchword in National Treasure.

Facebook Twitter Pinterest Robbie Coltrane: ‘I played it like Mount Rushmore.’ Photograph: Joss Barratt/Channel 4 Picture Publicity

Coltrane believes that his playing this role will provoke tabloid scrutiny of his personal history. “Someone asked me: ‘Do you not think this is a dangerous thing to do?’” he recalls. “Well, it would be if I was (a) a paedophile, or (b) a rapist. Being as I am neither, I have nothing to fear at all.”

Was he aware of that risk when he agreed to star in a drama inspired by Yewtree? “Of course. But I don’t care. I think it’s important to do this on behalf of the women who were raped, on behalf of all the people who were abused.”

He anticipates criticism from those who believe that real-life tragedy should not be turned into an entertainment of sorts. “As if a drama has to be a Sunday-night thing you watch with your eyes half closed. No, no, no. Drama has always illuminated what’s actually going on in society, in a way that the news or the internet doesn’t … The writing is absolutely beautiful. There’s total respect for people who actually were attacked. That’s the important thing for me. If you were raped, what is your life like thereafter, and what would it be like to stand up in court and talk about it? The moral strength required to actually say: ‘He fucked me,’ while he’s there in the room, and so is the public and the press.” Coltrane shakes his head in admiration.

Tim McInnerny plays Karl, Finchley’s best friend and former partner, in a successful comedy double act. The actor has personal reasons for feeling close to the material. “I was on Jim’ll Fix It,” he says, “teaching a 12-year-old girl how to do a speech from Shakespeare.” He pauses, gives a small nervous laugh. “She wanted to be Cleopatra. So they put her in a costume, gave her a set, and I coached her in this speech.”

This was in the early 90s, when he was at the RSC and still hot from Blackadder. Savile himself made little impression. McInnerny was thrilled to be on the show, placing a medal around the girl’s neck, because he remembered watching it as a child himself. Does he now feel uneasy about having taken part? “Yeah, absolutely. Ever since I found out about Savile. It makes you feel dirty.”

Facebook Twitter Pinterest The police case against Paul Gambaccini was dropped. Photograph: David Levenson/Getty Images

But, he says, the other side of the issue is when famous people are accused of these crimes and named in the media, yet no charges are made and the case is eventually dropped. “I mean, Paul Gambaccini is a friend of mine. His life was nearly destroyed. He was virtually under house arrest for a year and a half. He didn’t work, he couldn’t go abroad. And it’s a problem because it discredits the whole process, which you don’t want to happen because you want to find those guilty people. It’s a difficult area if the police take it too far with one particular celebrity.”

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In other words, McInnerny is playing a role he inhabited in real life – the friend of a man accused of historical sex crime. What does one go through in a situation like that?

“It’s very hard. You want to support him.”

And did you?

“Yes, as much as possible. Absolutely.”

So you chose to believe in his innocence?

“I suppose I did, yes. It never occurred to me not to.”

National Treasure looks closely at the contentious issue of “fishing” – the practice of leaking the name of an accused celebrity to the media in the hope that others claiming to be victims will come forward and thus strengthen the police case. Gambaccini and Cliff Richard, both of whom had investigations against them dropped on the grounds of insufficient evidence, have called for sex abuse suspects to remain anonymous until charged.

The writer, Jack Thorne, hopes that National Treasure will prompt a national conversation about these knotty issues. “You are potentially destroying someone by naming them in the media,” he says. “On the other hand, you are allowing people who may have been severely damaged to have the confidence and courage to come forward because that person is now in the spotlight. It’s so bloody complicated and difficult to know what the police and media should be doing, which is why talking about it seems important.”

While not a dramatisation of any specific investigation, National Treasure feels real and familiar in its exploration of the nexus of celebrity, sex and power. An illustration of its topicality: while filming was taking place in Bradford’s disused courtroom, five minutes away, in the present crown court, the footballer Adam Johnson was being sentenced to six years in prison for sexual activity with a 15-year-old girl.

Thorne’s research included interviews with police officers who had worked on investigations of celebrities suspected of historical sex abuse. He wrote the screenplay at the same time as the script for Harry Potter and the Cursed Child, switching between Death Eaters and true-life bogeymen with apparent ease. “Examining an issue that has horrified this country felt like an important thing to do,” he says. “It’s horrifying that Jimmy Savile existed. It’s horrifying that so many people seemed aware of it and we seemingly let it happen. The more we uncover about how much people in public life were aware, the more important it becomes to examine how we deal with this issue and what our responsibility is.”

As well as exploring the explosive impact of an investigation on the famous man at its centre, the drama looks at what it is like for his wife and daughter, played by Julie Walters and Andrea Riseborough, caught in the blast radius.

Facebook Twitter Pinterest Julie Walters in National Treasure. Photograph: Aimee Spinks/Channel 4 Picture Publicity

“Marie is the wind beneath Paul’s wings,” Walters says of her character. “Paul is a public entertainer, fragile in many ways, and Marie reminds me a bit of my husband, in that she is solid and there, and keeps things together, and supports him and looks after his fragile ego. She’s a Catholic and Paul isn’t, which I thought was interesting. I was brought up a Catholic; I’m lapsed. Paul has been unfaithful lots of times and she stays with him. I feel that the piece is about faith, and doubt and trust, and she makes that leap with both her religion and with him.”

In our voyeuristic gawping at a celebrity accused of such crimes, do we tend to forget how hellish it must be for their families? “Yes, but I think there’s also a curiosity. You do wonder about them and what they are going through. They are victims, in a sense, as well.”

Can she understand why wives so often stand by their husbands in these circumstances? “Yes, sort of. I think you have to believe in someone, otherwise what has 40 years of marriage been?”

National Treasure has been demanding for everyone involved. Unusually for television, there was a three-and-a-half week rehearsal period, almost as if preparing a play, to build the characters and their relationships. The actors speak with a fond shudder of exhausting 14-hour days filming this dark material.

“It’s been emotionally brutal,” says Riseborough, who portrays Paul and Marie’s daughter, Dee. “I’m playing somebody who suffers depression, and I’ve suffered depression pretty much my whole life. So that’s been difficult. Because I have been through it, I hope I might be able to tell the story. But it’s been really hard.”

Does she have to be careful not to trigger her illness by investing so much in the performance? “Yeah. I’m still learning.” She took the part because she trusts Munden, who had directed her in The Devil’s Whore. “I said to Marc that this wouldn’t be an easy thing, and sometimes I was going to need my hand held, metaphorically.”

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Much of the excitement around National Treasure is down to this being Coltrane’s first lead role in a television drama for seven years, and buzz that he is even better than in the glory days of Cracker. Certainly, his performance is entirely lacking in vanity – we see him walking with a cane, as he does in reality; we hear his laboured breathing; we see him half-naked. It feels exposing in every sense. At the same time, though, there’s a monumentality to his presence; the camera moves slowly around his face, exploring every crag. “I played it like Mount Rushmore,” he says.

Munden was keen to explore the tension between the “physical frailty” he detected in Coltrane and the actor’s supercharged aura. “What’s really interesting to me is the contrast between a man at this time of his life, like so many of the celebrities who have been prosecuted recently, and the potency and power and capacity to damage they must have had 20 years ago.”

Coltrane, at 66, remains an imposing figure despite having lost a great deal of weight in recent years in preparation for surgery on a bad knee. “I’ve got grooves in my cheeks that I haven’t seen since 1975,” he laughs.

What about the idea implied by the title that he himself is a national treasure? At this, the Rushmore brow descends so quickly that it seems we are in danger of rockfall. Does he not recognise himself in that phrase?

“Not yet,” he glowers. “That sounds kind of like being put on a shelf somewhere. Someone safe to be dragged out on national occasions and the Queen’s garden party. I’m not ready for that. No.”

He shakes his head. “I’m still fighting, boy. I’m still fighting.”