Many colleagues warned Robbie Coltrane against taking the starring role in National Treasure, Channel 4’s riveting new four-part drama about Paul Finchley, a much-loved veteran comedian whose life falls off a cliff when he gets a knock on the door one morning and is accused of an historic rape.

The 66-year-old actor reveals that, “A lot of people said, ‘Don’t touch it, Rob. It’s poisonous, it’s not going to do you any good.’ And then I got this script and I thought ‘Oh yes!’

“This story is so prescient. Every day a stone gets lifted, and somebody in the judiciary or Parliament or a medic is exposed. It’s just appalling. It appalls all of us. So I thought it was a really important piece of drama to do.”

But surely Coltrane must still have had some reservations about taking on such controversial subject matter? “No, no, no!” asserts the actor, in a manner that brooks no contradiction. “I believe with all my soul that it’s the job of drama to deal with things that the judiciary’s not very good at, the police aren’t very good at, the politicians aren’t very good at, the civil service aren’t very good at.”

The actor, who has also starred in Cracker and Harry Potter, continues that, “At the risk of sounding a bit pious – it won’t happen again! – the power of drama is that it can deal with subjects in a way that other things simply can’t.

“A lot of people said that about Cracker – ‘Why would you want to make an entertaining show about people getting murdered?’ But it’s not about entertainment, it’s about finding out how people get like that. That’s what the whole show is about.”

Julie Walters, who plays Finchley’s long-suffering wife, Marie, also had no concerns about appearing in this potentially contentious drama. “I didn’t have misgivings, because it’s truthful, there’s a truth behind it. So you can’t go wrong, really. It’s not sensationalising the issue for the wrong reasons. It’s being done for the right reasons.”

National Treasure, which is scripted by man of the moment Jack Thorne, also responsible for the hugely popular West End play, Harry Potter and the Cursed Child, homes in on the effect on Finchley’s home life. It explores the impact the accusation – furiously denied by Finchley - has on Marie and their mentally unstable daughter, Dee (played by Andrea Riseborough).

66-year-old Walters, who has done some of her best work with her friend, the late, lamented Victoria Wood, says she was drawn to National Treasure because she was so intrigued about why Marie would stand by her accused husband. “You do look at these real-life cases, and for me it's the wife who is most interesting. I want to know about her.”

The actress, who has picked up no fewer than eight BAFTAs in a long and distinguished career, carries on that, “It’s like when you see pictures of Rolf Harris and his wife - it’s her that I’m actually fascinated. How is she standing next to him, holding his arm?”

Walters acknowledges that, “The initial knock on the door must be devastating, earth-shattering.” But she goes on to outline just why she thinks Marie stays with a husband who has previously messed her about and is now in the eye of a tabloid storm. “Because she's a Catholic and wouldn't want to be divorcing him. Also, she has faith in him.

“And I could see why she did stay with him, from reading the script. The drama is about faith and needing to believe certain things. Marie won’t want her marriage to be over, despite his infidelities. It’s about forgiveness and trust, believing him when he says ‘I will always tell you what’s happening.’ Marie knows Paul's slept with other people, but he has always been honest with her. But this allegation of rape is of course a different matter.”

National Treasure also throws up the question of how much we ever really know about our partners. Walters, who scooped an International Emmy Award for Best Actress in 2009 for her leading role in A Short Stay in Switzerland, reflects that, “I think Marie chooses not to know everything about Paul.

“She chooses to turn a blind eye. And there’s a bit of a lack of intimacy about Marie, with her daughter and also with her husband. There’s a bit of a cut-off. Maybe there’s something in her history that we don’t know about. But for whatever reason, Marie and Paul are quite separate. They’ve almost worked out a deal: as long as she doesn’t know about it, or have to engage with it emotionally, then it’s fine.”

In addition, the drama examines the issue of whether people in Finchley’s position should be given anonymity. Coltrane concedes that the matter is, “Difficult. If you publish the guy's name, you may get genuine victims of abuse he carried out coming forward. Some of those people may be bogus – attention-seeking, trying to earn money from tabloids, delusional - but there may be genuine victims, too.

“But you also risk ruining a man's reputation if the allegations are false. In our case, seven people come forward, and it's up to the cops to decide who is telling the truth. Who do you want to protect, those who may be falsely accused or those who were victims? Somewhere along the line you have to choose between the two.”

National Treasure will inevitably prompt reflections on the shocking real-life case of Jimmy Savile, the DJ who got away with raping hundreds of victims. Coltrane says that, “I never met Savile, he was a creepy wee sh*te. You could just tell, couldn't you?! And so many people felt like that. How did he get away with for as long as he did? Because of the toxic culture of which he was a part. Hopefully, in this electronic, social media age, it wouldn't happen again.”

Walters and Coltrane are both widely-adored actors, and that status has only been enhanced since they both appeared in the Harry Potter movies.

Coltrane, who in 2002 was sixth in a poll to find the “most famous Scot”, behind the Loch Ness Monster, Robert Burns, Sean Connery, Robert the Bruce and William Wallace, says he continues to be surprised and flattered by the reaction of children all over the world to his performance as the gentle-giant groundsman, Hagrid. “It’s delightful really.”

The actor, who was personally chosen by the author JK Rowling for the role, adds that, ”I get the sweetest little letters from people. I got one a while back from Moscow, asking me to fly through the window and punch their dad if he was rotten to their mum. And often in Tescos, the mum will say to her kids ‘Do you know who that man is?’ And the kids will go, ‘You’re not Hagrid – you’re not nearly tall enough’.”

The actor, who won three BAFTAs for his role in Cracker, concludes by underlining what he hopes that audiences will take away from watching National Treasure. “I think it’s important to deal with these issues in some way and help people understand them. And hopefully to prevent it from happening again. Because you’re really doing it for the victims. A lot of people’s lives have been ruined by this kind of stuff.

“You have to do it right out of respect for them.”