Emily Thornberry and Stephen Mangan arrive at the Guardian’s London office at exactly the same time, with a similar air of frantic industry. Thornberry, sleep-deprived after a long night celebrating a Spectator award for comeback of the year, is in the middle of a packed day of interviews. Mangan is rushing between a meeting with his production company and another with Sky Arts, for whom he’s presenting a Portrait Artist of the Year show later this month.

Both Thornberry and Mangan studied law at university, but their careers took dramatically different paths. Thornberry spent 20 years as a barrister, before being elected MP for Islington South and Finsbury in 2005, with a majority of 484; she has since increased this to a walloping 20,263. Having served as shadow attorney general under Ed Miliband, she has held four posts under Jeremy Corbyn, including shadow Brexit secretary and, since June 2016, shadow foreign secretary.

Mangan, meanwhile, took law because he was told acting was an impossible dream. After graduating from Cambridge, he went to RADA. He broke into television as Adrian Mole in 2001, and went on to shows such as Green Wing, Episodes, The Split and his own production, Hang Ups. Nothing sums up his range better than his recent stage work: Mangan began 2018 as the terrifying intruder Goldberg in Pinter’s The Birthday Party and will end it by narrating Paddington stories at the Royal Albert Hall.

In person, they are not the odd couple you might expect. Thornberry likes to laugh, explosively, and Mangan knows his politics. Over the past couple of years, he has become a passionate critic of Brexit, a subject that has been causing the Labour party no small amount of angst. Taking their seats in an office overlooking one of the most radically transformed areas of Thornberry’s constituency, they brace themselves for the elephant in the room.

Emily Thornberry Are you going to start with the B-word?

Stephen Mangan Well, unfortunately. How fed up are you with talking about it?

ET [Laughs.] I’m fed up that it doesn’t go anywhere. We’re stuck in the same place we’ve been for a couple of years. We’ve just been watching the psychodrama of the Tory party, fighting among themselves and not looking at what’s good for the country. I’m fed up that we don’t really have any power.

SM We’re sitting here in your constituency. I don’t need to tell you it has some of the worst concentrations of poverty in the UK.

ET Sixth worst.

SM Is there any version of Brexit that will help those people?

ET Our biggest trading partner is going to have more difficulties trading with us. Obviously the economy is likely to get smaller, which will have an effect on taxes, which will have an effect on spending, which will have an effect on services.

SM So is there a moral justification for supporting, or even facilitating, Brexit?

ET I don’t want to be a part of undermining democracy in the UK. We had a referendum. I went up and down the country and said to people, “This is a really serious decision, we’re going to do what you say, think about it.” And they said we’ve got to go. So what do you do?

SM But the Brexit that was sold is not the Brexit that’s being delivered.

ET Well, I think people just voted to leave – not on any other details. So what you do is say, “52% said we should leave and 48% said we should remain, so the answer is we leave but we don’t go far.” That’s the only way you can really hold the country together.

What was David Davis doing in Europe? Buying Toblerones

SM The meaningful vote is approaching. I suppose in your ideal scenario the deal is voted down, an election is called and you get into power? What will Labour’s manifesto position on Brexit be?

ET We would say we should do as we have been instructed, which is to leave. We go back to Europe and say, “Look, let’s reset our relationship. Let’s be grown up about this. You know the difficulty we’re in, so let’s try and fix this.”

SM It takes two to tango. The EU have said there is no scope for major renegotiation – so how is your position different from the Tory position?

ET The difference is that Theresa May had this disastrous conference speech, where she put down so many red lines that there was no way they were going to be able to negotiate anything. Just think about David Davis. What was he doing going out to Europe? Buying Toblerones.

SM But isn’t there an inherent contradiction at the heart of Brexit? We’re looking for the benefits of the single market and the customs union, without belonging to them. If at the next election you said, “The policy we want to pursue will shrink the economy, lower GDP, lose us jobs,” you wouldn’t get close to office. But by supporting Brexit, that’s the policy you’re laying in front of people.

ET No, we would go into an election saying, “These negotiations have been a complete dog’s dinner and we want to negotiate on this basis…”

SM Over what timescale?

ET Well, when will the general election be?

SM But you need a plan.

‘I’ll be an estate agent next.’ Photograph: Suki Dhanda for the Guardian

ET OK. So Europe has said they’re not going to extend article 50 unless there’s a major change. Well, having a new government is a major change. I think there’s a strong argument for us extending article 50 if we have to. I also think – and it’s not Labour policy – there’s an argument for us saying we’re going to do the best we can, and if at the end of that it’s to the advantage of the country to ask people what they think of it, then we can go for it.

SM Why not do it now?

ET The polling is not emphatically in favour of remaining. I would have thought by now the public would be at least 70% in favour, given what they’ve seen. And I think the reason they haven’t changed is because what they hear is, “You’re stupid, you’re racist, you’ve been manipulated, your vote wasn’t valid” – and they feel insulted.

SM The Labour party’s overwhelmingly in favour of the People’s Vote.

ET Well, Labour party members are overwhelmingly in favour of remaining in the EU. And we can’t if we don’t get an emphatic vote in favour. Let’s say we have a vote and they say again, we have to leave.

SM Then we leave.

ET No! Because the sort of leave we’d then have would be appalling! It would be a licence for the hardest possible Brexit.

SM But the British public would have made that decision with their eyes open. Not after a campaign of lies, half-truths and possible illegality.

ET There’s one thing I’m genuinely worried about. What kind of campaigning would Farage do? What would the hard right do? And people saying: “We told that London elite that we wanted to leave. They’re not listening to us, we’ve got to make it even clearer.” I think it would be even more divisive.

Can you see yourself supporting a second referendum? Yes or no, Emily?

SM But you’re facilitating one of the biggest acts of self-harm this country’s ever seen.

ET But it’s something the country wants.

SM Well, that’s a matter for debate. Theresa May went for her big mandate and didn’t get it. And with Labour’s ambiguity during that election, trying to appeal to all sides, there seems to be a lack of clarity. I can see, politically, why that might be useful – but isn’t now the time to lay out what we think is right and proper?

ET If I was confident that a referendum would result in us remaining and there not being division in the country, I would be absolutely fine about it. But public opinion seems to have shifted only a couple of percentage points. It’s really hard, because I do want to remain and my constituents want to remain. But it’s a question of trying to keep the country together.

SM Could you see yourself supporting a People’s Vote?

ET Well, our policy…

SM No, not your policy. Could you personally see yourself supporting it? Yes or no, Emily? [Laughs.]

ET Well, what’s the referendum question?

SM Is there any form of referendum that you could see yourself supporting? You’re not going to answer me, are you?

ET If there was one like before, with leave and remain, then I would vote remain. But I don’t think that’s going to be the nature of the referendum. So I don’t know. We want a general election, we want a chance to negotiate this properly.

SM I’ll stop grilling you on Brexit now. At least it’s taken our mind off Trump. You’re probably going to be foreign secretary soon. How are you going to cope with that chump?

ET I think you just have to work around him. He’s a bully and there’s no point in trying to kowtow to him or hold his hand. The midterms were really encouraging. He’s the first president whose approval rating hasn’t gone above 50%. He’s provoked a lot of women to go into politics. So the swing back is really interesting. I’m hopeful that he won’t get another term.

SM But the us-and-themness of politics now is really dispiriting.

ET It is. So, last night I was outside the building where your “wife” worked in that series you did about lawyers [The Split]. I was going: I know that building!

SM That’s the great thing about being an actor. I get to be a lawyer for a few months and then go back to being something else.

ET And you studied law?

SM Yeah. You?

ET I did 20 years as a barrister. Largely crime – actions against the police, inquests, anything with a jury, really.

SM I was always told at school, no one gets to be an actor, so be a barrister and you get to stand up and do a bit of acting. Is that true?

ET It’s a bit like being an actor. Your clerk is like your agent: you’re only good as your last case, and there’s always this feeling of insecurity, that there may not be another case.

SM For me, law was about pulling the emotion out of a situation and looking at it more dispassionately.

ET It’s about painting pictures, telling stories, talking about what motivates people. Going into politics, there’s lots in common.

SM You’ve chosen two really well-regarded professions.

ET [Laughs.] Yeah, I’ll be an estate agent next.

SM They’re both quite combative. Do you like an argument?

ET Yes.

SM As an actor, you’re paid to have a certain amount of emotional vulnerability, but the job can be quite wearing on your psyche – the rejection and being judged. So you have to develop a thick skin but be as open and sensitive as possible.

ET Don’t you compartmentalise? If I get criticism from my friends or family it can be devastating. Those who don’t know me, I don’t give a flying fuck. There’ll always be the haters. You must have the haters.

SM The older you get, the more you don’t care. The “stick to acting” thing is a non-starter. I’ll stick to acting if people stop having opinions about TV programmes.

‘How many laughs are there in a shadow cabinet meeting?’ Photograph: Suki Dhanda for the Guardian

ET [Laughs.] Very good.

SM Obviously actors have big profiles. Sometimes you just feel like you can’t sit there and say nothing.

ET It can be helpful to a cause if a big name signs up to it. I have lots of actors in my constituency. If a famous constituent stands up and says something and other people get involved, what’s wrong with that?

SM You’re seen as a possible future leader of the Labour party, if they ever get around to electing a woman…

ET [Laughs.]

SM Do you think about that? We all imagine that politicians have half an eye on policy and ethics, and half an eye on their careers.

ET I tend to live in the moment.

SM So there’s no plan? That’s hard to believe.

ET You do things as well as you possibly can. I shadowed Boris Johnson. He spent all his time thinking about being prime minister and he was an absolutely appalling foreign secretary – the worst we’ve ever had. Don’t you have the same with your career?

SM You can’t plan a career in acting. The jobs you turn down can be as important as the jobs you take. My criteria is always: would I watch this? If I wouldn’t watch it, then I don’t do it.

ET Lots of your stuff uses improvisation, doesn’t it? I love the idea, in Hang Ups, that you have no idea what the actors are going to say. How do you keep a straight face?

SM You don’t always. Actors are there to replicate a genuine instant reaction. That’s not easy when you’ve read a script 15 times, but when you’re improvising it’s very easy. I think humour is the best way of dealing with any human experience. If you have a sense of humour about yourself, you have a sense of perspective. How many laughs are there in a shadow cabinet meeting?

You say the wrong thing and – boosh! – it’s reported everywhere

ET [Laughs.] Actually, we do laugh.

SM Who’s the funniest?

ET Well, some people are funny but they don’t mean to be.

SM Who?

ET I’m not saying!

SM Is John McDonnell a big gag man?

ET No, I don’t think John’s a big gag man but his deputy [Peter Dowd] is. I think the whole point is, you can have a sense of humour and be sincere.

SM Sometimes you’re talking off the cuff and I imagine that’s a high-wire act because you say the wrong thing and – boosh! – it’s reported everywhere. I can see why politicians fall back on certain rehearsed responses, but I can also see why the public gets frustrated.

ET Yes. The public want us to be authentic and real, but they also want us to be perfect. I do think it’s easier now that Jeremy’s leader, because he is authentic and that’s quite empowering.

SM Well, that’s partially true. You get the impression that he is a man of principle, but you don’t always feel you’re getting the full picture. Like the whole antisemitism row. It took a long time to deal with. People start to question if he’s genuine in those pronouncements.

ET Yes, I think we didn’t deal with that well. We were like animals in the headlights.

SM That has a knock-on effect. People think, if you can’t deal with a few cases of antisemitism over a period of years, how are you going to solve Brexit in six months?

ET I hear you. We are dealing with it.

SM Is there a distrust of the Tony Blair-style smoothness within this Labour party?

ET Yes.

SM Because that was so effective, but was also seen as possibly disingenuous and manipulative.

ET Cameron took a lot of his style from the way Tony Blair did things, and I think people, after a while, just don’t trust that.

SM I’ve played Tony Blair twice. It was an absolute joy, because he always gave the impression of being aware that he was the star in his own movie. He knew he had charisma.

ET We moved into our house on the same day as the Blairs. Four lawyers moving into the same crescent on the same day.

SM What sort of neighbours were they?

ET I’d see him with the kids in the playground but I didn’t want to go up to him and say the 1990s version of “Can I have a selfie?”

SM I look at Cameron now, in his million-pound garden shed, and he doesn’t know what to do. Tony Blair, too. What do you do next?

ET I didn’t get into politics until I was 45. Cameron had nearly retired by that age. I’ve already done being a barrister and being a mum.

SM How old are your kids now?

ET Two of them are in their 20s and one’s 19.

SM What do they make of you being such a high-profile figure?

ET I think they find it a bit embarrassing.

SM “Oh, Mum! Stop making speeches in the Houses of Parliament! Back a People’s Vote, Mum!”

ET [Laughs.] …and we’re back!

• This conversation appears in the Guardian’s Weekend magazine and has been edited and abridged for length.

If you would like a comment on this piece to be considered for inclusion on Weekend magazine’s letters page in print, please email weekend@theguardian.com, including your name and address (not for publication).