Emma Thompson on her latest film role in The Children Act

Emma Thompson is famously un-starry but she is also an Oscar-winning pillar of the British acting establishment and regarded as a national treasure.

As a result, one wants to address her with as much respect as one can muster. It is therefore more than slightly horrifying to realise you have casually called her ‘Emma’ rather than acknowledging the Damehood she received earlier this year in the Queen’s birthday honours.

“I will let it go this time,” she jokes mischievously. “But next time – off with your head. The executioner is in the wardrobe.”

The star of such films as Howards End, Love Actually and Sense and Sensibility in her latest film, The Children Act, Thompson plays a High Court judge.

Adapted by Ian McEwan from his novel of the same name, the project offered Thompson, who is now 59, a character she had never seen before.

“When you hear the word ‘judge’, even in this day and age, in our patriarchal way you just tend to think, ‘Oh it’s a man’,” she says.

“We have never seen a film about a woman who is a High Court judge, I have never seen that, and so I was fascinated to do the research and see what it’s like to be that very rare thing of a woman who has that kind of power. She has to make decisions that are life and death, decisions that affect families and people in visceral, extreme ways. Feeling that power in the corridors of the Courts of Justice was overwhelming. I made friends with two judges who I love and admire more than I can begin to tell you. They sit above everyone else and it makes the court into a theatre and they are the one who everyone is looking up at.

“You can also see everything that is going on and the women used to say to me that they watch the body language a lot, it’s a completely compelling job.”

Thompson spent a great deal of time “backstage” in courts observing their arcane procedures and marvelling that only judges and their clerks can walk on the red carpet of the corridors.

“You suddenly realise that they are these god-like creatures in their robes, walking around with this extraordinary power, and hardly any women have that kind of power in the world, ever.”

In the film, Thompson’s character Fiona Maye must rule in a family court on whether a hospital can treat a seriously ill teenage boy, played by Dunkirk actor Fionn Whitehead, against the wishes of his Jehovah’s Witness parents.

“The thing to note is that the family court is considered the poor cousin,” she says. “Criminal is the macho guy thing, that is where the ‘real’ stuff happens, and that view is just typical, frankly.

“But family court is where the real drama and the real pain is, it’s terrifying. All human life is there.”

Thompson said that fact was drawn into particularly sharp relief during the case of Charlie Gard, the baby whose fate was the subject of a protracted court battle after he was born with the rare genetic disorder mitochondrial DNA depletion syndrome.

His parents fought a five-month campaign to have him transferred from Great Ormond Street Hospital in central London, where he was on life support, for experimental treatment elsewhere before he eventually died in July 2017. “You realise here are no winners,” Thompson said. “There is no right and wrong really.

“There are decisions that have to be made and they are always painful and they are painful on both sides and they are painful for the judge. Sometimes in some human situations there is only pain and that is how it is. We are so used to needing this sense that it’s got to come right in the end because we tell stories in order to make ourselves feel better about the fact that sometimes in life there is only pain.”

Thompson is more optimistic when it comes to the subject of women in positions of power, especially on the judiciary.

She feels it is “terribly important” for young girls to see characters such as the female judge depicted on screen so they know that kind of job is possible for them. “As soon as you see a powerful role being taken by a woman, you can see that and think, ‘Oh, I can be a judge’.

“But that has to be backed up. It’s alright having dreams but they have to be backed up by the intentions of your government. If they are cutting educational opportunities for people, in particularly those from lower income environments, the pool from which women are going to be able to come up to become judges is getting smaller and smaller.

“This is all about educational opportunities, and also about challenging the automatic routes that mostly men, mostly white men, can take through Eton and Oxbridge. The judiciary is like parliament, it’s still a club for straight, white men – although I have to say that is changing.

“There are little pockets of hope and the greatest place to find hope about all of these issues is with the young generation. Go to the Edinburgh Festival, go to the little theatres – go and see what poor, young, female writers are writing about, what it is to be born into the female body today.

“Watch Jill Soloway’s The Female Gaze and watch Nanette, Hannah Gadsby’s piece on Netflix, inform yourselves and see what is going on.”

With those rousing instructions, I stand to take my leave, ducking so I don’t hit my head on a low-hanging light.

“I’m so glad you’re going out while bowing,” says Thompson with a cheeky grin. “If you go backwards, that’s even better.”

The Children Act is released in UK cinemas today.