Early last week, for the briefest of moments, the tensions swirling around Scotland’s relationship with England and the deepening UK crisis of Brexit were perfectly symbolised by Boris Johnson’s arm.

Amid the angry noise of a crowd of demonstrators, Johnson was on the steps of Bute House (the Edinburgh residence of the Scottish first minister) exchanging the usual opening pleasantries with Nicola Sturgeon for the benefit of photographers and TV crews. Then came the moment when everyone had to go inside – whereupon Johnson rather condescendingly extended one of his upper limbs, with a view to escorting Sturgeon into her own HQ. She appeared to say something, whereupon the arm went down and Johnson awkwardly walked in, a moment that inevitably went viral.

“I don’t think I said: ‘Put your arm down,’” she says. “Those kind of things happen in a flash. But I am aware that he was trying to do something that some people would describe as chivalrous and other people would say was a subconscious powerplay. I was kind of aware of him trying to … usher me in.” She smiles. “And I think, in the instant, I thought: ‘That’s inappropriate – you go in first.’ But these things happen in a nanosecond. It wasn’t at all deliberate on my part.”

One thing, however, was certain: she did not look happy. “Again, you’ve got to remember that these photos are taken in an instant,” she insists. “But, that said, I abhor what he is trying to do to the UK. And I abhor what the implications of that are for Scotland. So I don’t think it will be surprising to anyone to hear me say that I wasn’t absolutely thrilled to be welcoming Boris Johnson as prime minister.”

‘I don’t think I said: “Put your arm down.”’ The incident outside Bute House. Photograph: Jane Barlow/PA

Before that meeting, they had crossed paths once before – at the VE Day celebrations immediately after the 2015 general election, at which Johnson brusquely asked Sturgeon if granting the Scottish government full control over its tax and spending powers would silence calls for independence. “We walked together through Horse Guards Parade. And I vividly remember, he said something like – and this is not a direct quote – ‘What would buy you off? Would full fiscal autonomy buy you off?’ I said: ‘No, not really.’” She laughs.

Four years on, she conveys a similar sense of Johnson reducing complex questions to breezy inanities. “Talking to him, there’s a sort of surreal element to it,” she says. “He talks about Brexit, and a no-deal Brexit in particular, as if there is nothing to worry about, and anyone who says there is just being pessimistic and downbeat. I was very frank with him that I think it would be catastrophic, if it were allowed to happen. But, you know, then there’s just a waving of the hands, and: ‘No – it’ll all be fine, and anyone who says otherwise is just indulging in doom and gloom.’ I described it after the meeting as dangerous. ’Cos it is dangerous.”

One other aspect of Sturgeon’s discussions with Johnson is fascinating. To quote a profile of her published a few years ago, Sturgeon is “state-educated, working-class, left-leaning and believably human”. Johnson, who, by contrast, swaggered to where he is along a familiar path that passed through Eton and Oxford, has created his borderline absurd persona with the kind of chutzpah that tends to be the preserve of the truly posh. Class, I suggest, must have been a constant subtext in their exchanges.

“I do think there’s an element of …” she pauses. “Well, I said a moment ago that he talks what, in my view, is a load of nonsense, with utter conviction. And I do think there was an element of that sense of entitlement; the idea that you can make anybody believe anything if you say it in a particular way.”

Sturgeon became first minister in late 2014, taking over from Alex Salmond after the referendum on Scottish independence was won by the no campaign, but her party was still in the midst of its imperial phase; it would go on to win 56 out of 59 Scottish seats in the 2015 general election.

She is one of those politicians whose voice and mannerisms seem instantly familiar: the precise, sometimes overlong, distinctly lawyerly sentences; the way that even her most speech-like utterances contain enough everyday humanity to keep you interested. Her humour, when it enters the conversation, gives a subtle sense of someone who appreciates the ridiculousness of politics, particularly at a time like this. But, unlike Johnson, she is also at pains to point out that Brexit is deadly serious, and that Britain faces “the most rightwing government we have had in my lifetime, probably including the Thatcher governments”.

We talk for an hour in one of Bute House’s meeting rooms on the same day that the Bank of England is saying that there is a one in three chance of a Brexit recession, and Sky News has published a wad of civil service documents warning that a no-deal exit could trigger “potential consumer panic and food shortages”, “operational gaps” in policing and national security, and “law and order challenges” in Northern Ireland. Given that the UK was meant to leave the EU on 29 March, Sturgeon says that she and her ministers have been grappling with this kind of stuff for a long time. “A lot of the focus has been on continuity of transport – particularly ferries, and making sure we’ll have supplies of food,” she says. She also mentions “adequate supplies of medicine”, among “many and varied aspects of what this will mean for people’s lives”.

I think the abdication of leadership on the part of Jeremy Corbyn right now will be the stuff of history books

If he were here now, I suggest, Johnson might assure her that whatever the disastrous projections of a no-deal Brexit, and however much time and money has been spent preparing for one, it is a negotiating ploy. She half agrees, but insists that whatever he and his allies say, Johnson’s reckless approach to leaving the EU is pointing to inevitable catastrophe.

“I’m not breaching confidences here because he’s saying this publicly: he’s saying he doesn’t want a no-deal Brexit, and he doesn’t think that’s where we’ll end up. But that’s part of the delusion in his position. He has set a negotiating position that everybody knows is unachievable: getting rid of the backstop in its entirety, getting rid of the withdrawal agreement in its entirety and starting all over again. I don’t think there’s anybody outside his circle and the hangers-on around it who thinks that is remotely achievable. So, if that’s your starting point, where do you go from there, other than over that cliff-edge?”

The prospect of this calamity is a big part of Sturgeon’s other focus of attention: the SNP’s renewed push for a second independence referendum. It’s her second since the Brexit vote: in 2017, she announced her intention to hold another Scottish referendum in 2019, only to retreat. Now, she says, the prospect of the UK finally leaving the EU – and the distinct prospect of us doing so in the most damaging way possible – makes the case for a vote unanswerable, even if senior Tories from the prime minister down have made it clear that they will not grant her government the relevant powers.

She has said she wants a referendum on independence by 2021 and that the second half of next year would be the right time. “That’s what I intend, yeah.”

So what happens when the Westminster government says she can’t have one? “Well, look, if we get to that bridge, I will set out how I intend to cross it … We’ve got legislation going through the Scottish parliament that is necessary for a referendum on that timescale. When we get to the endpoint of that legislation, we’ll then have that discussion with the UK government about the transfer of power.”

Will it be a discussion? Or a historic clash? “Let’s wait and see, OK? And then I’ll set out how we intend to deal with that.”

After her meeting with Johnson, she said he hadn’t been “at his most comfortable” when talking about the case for a second independence vote. What did she mean by that? “He didn’t have much of an answer. He can defend the union and oppose independence: that’s entirely legitimate. He kept trying to get back to the substance of the argument about independence. And the point I made back to him was: ‘Let’s have the debate on the substance, but let’s have it out there and let the public decide.’ I suggested we might do a live TV debate on it, at which point he decided it was time to go,” she says laughing. “Or his advisers decided it was time to go.”

Sturgeon with Alex Salmond in 2013, launching the Scottish government’s white paper on independence. Photograph: Murdo MacLeod/The Guardian

In 2014, political discourse was already smattered with fake news and polarised nastiness, and, although most of the debate around Scottish independence was substantial and serious, the independence referendum campaign and its aftermath were no exception. Before and after, for example, I met lots of people who had read online that the vote was rigged, and seemed to believe it, and occasionally witnessed aggression that could explode in an instant, online and in the real world. Another Scottish referendum, I suggest, could potentially be a horrible, rancorous mess.

“I don’t think it has to be. I don’t think it’s inevitable at all. The kind of tone and tenor of political debate, and I’m not just talking Scotland here, I’m talking globally … of course that concerns me. I think it should concern every right-thinking politician. I think one of the challenges for politicians who don’t want politics to continue in that direction is that we have got to be prepared to stand up for a reasoned, logical and respectful way of political debate.”

But do people even listen? It’s out of her control, surely? “But the alternative is a counsel of despair … that’s not the kind of politician I am. I think there’s a need for politicians who actually value a type of political debate to stand up for that, and defend it, however difficult and, at times, seemingly hopeless that may be.”

A little later, she expands further: “Without saying that anything will ever be perfect, if you contrast the independence referendum with the Brexit referendum, they were worlds apart. You had an electorate in Scotland in 2014 that was highly informed, engaged and knew what they were talking about. That wasn’t the case in 2016, when the equivalent of the 800-page independence prospectus was the lie on the side of the bus.”

There is one aspect of the immediate future that we can only talk about in the most minimal way. At some point next year, Salmond will go on trial on two charges of attempted rape, nine of sexual assault, two of indecent assault and one of breach of the peace – allegations that he continues to strongly deny. Inevitably, the outcome of any Scottish referendum partly hinges on what happens, doesn’t it?

“No, I don’t think it does. I don’t think it does. We’re talking here about the future of a country. But, for reasons you will understand, it’s not something I’m able to talk about in any more depth or at any more length.”

Does she expect to be a participant in the trial? “Look, it’s a criminal process. It would be utterly inappropriate for me to say anything about it.”

Leaving aside another independence referendum, the big event that may well materialise very soon is a UK general election. In 2017, amid talk of public weariness about another independence referendum, the SNP lost 21 Westminster seats. Now, she thinks the prospect of Brexit would make things very different. South of the border, I remind her, there is a lot of talk about looking on the bright side of Labour’s dismal poll numbers: the idea that they could at least win enough seats to strike some kind of deal with the SNP and take power that way.

Sturgeon talking to Jeremy Corbyn in 2016. Photograph: Andrew Matthews/PA

“I have fought two general elections now as SNP leader, and, in both of them, I have been pretty candid. We would always want to be part of a progressive alternative to a Tory government. That remains the case. That said, I’m no great fan of Jeremy Corbyn. I think his lack of leadership on Brexit in particular … well, if we do crash out without a deal, he will bear almost as much responsibility as Theresa May or Boris Johnson. I can’t see the SNP going into formal coalition with Labour.”

Does she rule it out? “Look, I think in politics you’ve got to be careful. But it would not be my intention, to go into a formal coalition. I said that in 2015 and 2017 – that’s not a new thing. But some kind of progressive alliance that could lock the Tories out of government … it wouldn’t be a blank-cheque type scenario. We would want Jeremy Corbyn to take a very firm anti-Brexit position. We would look to do what was right for Scotland.” (If Brexit were somehow cancelled, Sturgeon has said that the possibility of an independence referendum would remain because “Scotland needs to keep its options as open as possible.”)

Our time is running out: among Sturgeon’s engagements today is a Scottish cabinet meeting, which will spend “a fair chunk of time” talking about no-deal planning. I have only one more question: what would be her advice to someone who lives in England? “Move to Scotland,” she says, with a quiet laugh.

Is she at least half-serious? “More than half-serious. We need more people in Scotland. Absolutely: move to Scotland.”

But if someone can’t, or won’t? “It’s very difficult. And I don’t have an obvious answer to that. I think the abdication of leadership on the part of Jeremy Corbyn right now will be as much of the stuff of the history books as what’s happening in the Tory party. And I think it’s tragic. If you’re in Wales, vote for Plaid Cymru, obviously.”

She pauses, before she issues a few last sentences that come close to sounding like a compact speech. “Throughout my political career, I’ve had – and I don’t mean this pejoratively – people like you almost portraying Scottish independence as some kind of betrayal of the left. I can sort of understand it. But the answer to that can’t be that Scotland just languishes with the rest of the UK and gets taken down the wrong path. I think the best thing Scotland can do for people who want a different path for England is to lead by example. We can show it can be done.”

• This article was amended on 6 August 2019. An earlier version incorrectly said Boris Johnson had entered Bute House before Nicola Sturgeon.