SOME years ago, Susan Calman was at her school reunion in Glasgow. “Somebody came up to me and said: 'I don't understand it Susan, you weren't even funny' … So I think it probably came as quite a surprise to everyone that this is where I've ended up.”

Right now, “this” is her local boozer in Strathbungo, Glasgow. It's one of those howffs you peer into before venturing over the threshold. (When there's a burst of shouting later, Calman tells me it isn't a fight, just the punters watching a horse race on telly. “This is the place where the old barman would pop across to the bookie for you if you were too pissed.” It's also where she and her wife, Lee Cormack, had their post-wedding knees-up. The charming, uber-calm Mrs Calman is with us today, sipping a beer and listening quietly throughout.)

But Calman isn't actually talking about the pub. She's being more expansive. Her “this” refers to her career as a comedian, writer and broadcaster and to the celebrity she has acquired as a result. And where has she ended up? Everywhere. On your tranny, thanks to radio shows such as Susan Calman Is Convicted, The News Quiz and Listomania, which she hosts. On your television, as a panel show guest or in sitcoms like Fresh Meat and Rab C Nesbitt. On a stage near your home courtesy of her touring stand-up shows. Even on your mobile or laptop: 61,000 Twitter followers and counting.

The 41-year-old may also arrive on your bookshelf soon. On Thursday she published Cheer Up Love: Adventures In Depression With The Crab Of Hate, a book about the causes and effects of depression and a candid and often laugh-out-loud-funny account of her long battle with it. Actually, hand-to-hand combat might be a better description: her story certainly has its life or death moments. The crab of hate, by the way, is her crustacean version of the more familiar black dog.

“What I didn't want to do was write one of those tragic life story books that you see, which are just heartbreaking. I want people to enjoy reading the book and what I've found about comedy is that if you make people laugh you can tell them things that might otherwise be uncomfortable,” she says. “So I very much wanted to make it a fun book, but also an honest book because to try and explain exactly what the depression is like, you have to be quite honest about it.”

Calman's “crab” has sharp claws and a long reach – back through her 20s to her teenage years and into her childhood. She has talked about her depression before, on stage and in an early episode of Susan Calman Is Convicted, but it's only in Cheer Up Love that she reveals how long-standing the condition is and how serious it became. As a teenager she would self-harm and things reached a point where, aged 16, she attempted suicide with pills and was sectioned.

“I think the fact that I was in hospital when I was 16 is probably something that not a lot of people would know about. And just how long I've struggled with it [depression],” she says. “When you see the kind of detail about how long it has been going on for, it has a bit more impact than just listening to a radio show about it. So certainly how depressed I was when I was younger is not something people would know.”

In the book she keeps the details of the suicide attempt brief, though there's a winning quip about the Glasgow heckler whose response to a rare on-stage mention was to ask: “Did you manage it?”

In conversation, Calman takes the same course. “I still find it difficult to talk about,” she admits. “So I wanted to mention it but I don't want it to be some sort of depression porn. It's not a poor-me [thing]. It happened. It's something which is part of my life, it's something which I feel almost embarrassed about sometimes. So I didn't want to get into it in too great detail.”

Writing the book was difficult, she says, that chapter in particular. But there's no sense that Cheer Up Love is a deliberate act of catharsis or an attempt to hasten any kind of healing process. Instead Calman presents herself as a woman who has already come to terms with who she is, and the book ends on a broadly positive note. “My crab of hate is still there,” she writes, “except now I’ve bought it a hat. A top hat, in fact, and it sits at a jaunty angle.”

But you don’t have to rewind too many years before she’d have been unable to pen those words. I ask her how many.

“I think probably starting doing comedy really changed things because I felt tremendously free, even though I was earning no money and it was pretty awful at times,” she says.

This is a decade ago, then, when she had already turned 30. “Then in 2010 it kind of clicked and personally also kind of clicked in terms of becoming more confident about who I was. In the early days I wouldn’t have had the guts to admit anything really.”

Her breakthrough year was the 2012 Fringe, when she performed This Lady’s Not For Turning Either, a show about equal marriage. It brought her full houses, critical praise and – perhaps not unconnected – a moment of realisation: “I thought, ‘I know what kind of comedian I want to be and I don’t mind being honest about depression and how I feel about things’.”

The year 2012 was also the year she and Lee were married after nine years together.

“We did it the first appointment we could get as soon as it became law. It wasn’t to prove a point, I just wanted the same rights that my brother and sister had had when they got married.”

And have they ever considered having children? It’s the only time Lee speaks up. “It was something we toyed with superficially a number of years ago and then decided absolutely not, that it wasn’t for us,” she says.

Calman jumps in. “Got my niece and nephew, you have your niece [this to Lee], got friends with kids.”

Children’s TV, on the other hand, is a different issue. Calman has previously presented shows on CBBC and is currently filming a new, 30-part series in Glasgow for the channel called Top Class. “It’s University Challenge for kids,” she says. “I like doing kids’ TV so that my niece can see me on the television.”

Her niece is four, her nephew one, the children of her elder sister who lives in London where she works as a doctor of nursing. Calman’s elder brother still lives in Glasgow and works for the police. Her father is Sir Kenneth Calman, the former Chief Medical Officer of Scotland who headed up the Commission on Scottish Devolution. Initiated in 2007, it came to be known as the Calman Commission.

Another subject Calman tackles in some depth in the book is social media. Mostly, she says, her experience is good. “The majority of interactions are very positive. I think you get quite caught up in stuff that happens, but I don't find it a problem to be honest.”

But naturally she tackles the ills as well as the benefits, and online bullying and its distressing effects are one of the greatest of those ills. “I think you have to find a way of navigating your way through it. If you want to have a fight on social media you can. If you don't, then you don't.” (That comment, by the way, accords with number seven on her How To Keep Your Head Safe On Social Media list. Three is “Don't feed the trolls” and four is “Never Google yourself”. Number one is “Never post a picture of an otter”, but that's a long story).

Calman has had her own experience of online bullying and its distressing effects. It started after a perfectly legitimate satirical skit on the subject of the Scottish independence referendum on The News Quiz and became, in her words, “a s***storm”. Two years on, she's keen to draw a line under it. “It was a very emotional time,” she says. “People were debating very freely. People I think sometimes were over-zealous in what they said. I think we've all moved on from that.”

What Susan Calman moved on from she decided to become a comedian was a career in law. She would quibble with the description, but she was a high-flyer.

She studied at Glasgow University after leaving the fee-paying High School of Glasgow and it was as an undergraduate there that she heard a lecture by Sister Helen Prejean, who campaigns against the death penalty in America and who features in the film Dead Man Walking (she’s played by Susan Sarandon). “One of the most impactful afternoons of my life” is how Calman describes it.

Inspired, Calman applied for and won a scholarship to travel to North Carolina to study the death penalty. And so, aged 20, she found herself walking onto Death Row “to talk to a man who killed several women. Just me and him”.

That wasn’t the only eventful episode. Later, in a Holiday Inn in Virginia, she had a pistol pointed at her under the table, held by a prison guard accused of having had sex with a prisoner. On a visit to a trailer park, a shotgun was waved in her direction.

Back home, the reality of life as a Scottish lawyer was a little different. “You have high hopes for what you’re going to do as a lawyer,” she says, “and then you end up doing a telecoms line for the contract for the Edinburgh trams.”

Today, the capital’s shambolic transport infrastructure is the sort of thing she might poke topical fun at on The News Quiz. She’s a regular voice alongside fellow panellists such as Jeremy Hardy, Rebecca Front, Andy Hamilton and Mark Steel.

Calman speaks warmly about Sandi Toksvig, former presenter of The News Quiz, new presenter of QI, co-founder of the Women’s Equality Party and a mentor of sorts. “Generous”, “tremendous” and “brilliant” are all wheeled out. Calman is equally effusive about The News Quiz itself. “It’s still the show that’s dearest to my heart because it’s where I started with Sandi.”

We talk radio for a bit. To my jaundiced ear, much of BBC Radio 4’s post-PM teatime output is execrable: snobbish humour dominated by Oxbridge types which seems screamingly out-of-step with modern Britain, though I don’t phrase it quite like that.

Calman steps up anyway. “I’m absolutely not saying that isn’t there, but my experience has been that I have never suffered because I went to the University of Glasgow. I have never been treated differently. I think BBC comedy has been through a cull, a change, which means that the things they are commissioning now are very different perhaps to what they did before.”

She points to the number of women who have been given important early exposure through slots on BBC Radio 4, among them Sarah Millican and Sara Pascoe. She points to new voices such as Nish Kumar, presenter of BBC Radio 4 Extra’s late night Newsjack. She points to the influence of women like commissioning editor Caroline Raphael (now departed) and Julia McKenzie, recently-appointed Executive Editor for BBC Radio Comedy.

“The new raft of commissioning, I think, is very, very bold. Caroline Raphael turned me down about six times but she was right. The shows I suggested were s***.

“The only thing that does annoy me about being on Radio 4 is that you don’t ever hear young comics saying they want to be on radio because they see being on television as the most important thing.”

As a result, they miss out on a potential audience of millions. “I would take them any day above being on a show on ITV2 that has 100,000 viewers,” she says.

Calman’s 2016 Fringe show is to be called The Calman Before The Storm. As a way of marking 10 years at the world’s biggest arts festival – proving ground for many generations of comics, herself included – she’s themed it around the things people have said about her over the years, culled from reviews, interviews and, yes, social media.

Expect “Why’s this fat dyke taking up half of my television screen” to feature. That’s one of the numbskull Tweets she recounts in Cheer Up Love. It’s a there-to-here journey through her own past. Or, in her words, the story of “how I’ve ended up from the first Fringe to this”. And there’s that phrase again.

So much for where she’s ended up. Where is she headed to? Who knows. But stay on the current trajectory and she could soon acquire that strange, quasi-national treasure status that radio and television can confer. It has already made a heroine of Sandi Toksvig, so why not her protege? “One day I will be in charge of everything and then we’ll see what happens,” she says. “It’ll just be me and cats for half an hour.”

Bring it on.

Cheer Up, Love: Adventures In Depression With The Crab Of Hate is out now (Two Roads, £14.99).

Susan Calman is at the Boswell Book Festival, Dumfries House Ayrshire, today (3.30pm)