THERE is only one letter of difference between brother and bother, a theme which, as Neil Forsyth will tell you, plays out with calamitous effect in his latest television project.

When working on the idea for BBC Scotland drama Guilt, the Dundee-born writer deliberately sought to explore what he views as one of the most complex human relationships: siblings.

The four-part series, which begins this month, centres on two brothers – played by Scots actors Mark Bonnar and Jamie Sives – who find their lives turned upside down after failing to report a hit-and-run on the way home from a wedding.

"I wanted to write about siblings," says Forsyth. "For me, it is the most interesting, dramatic relationship because you have two individuals who can be very different and maybe people who wouldn't naturally spend time together.

"Yet they are trapped together in this lifelong way with a vast emotional hinterland and history behind their relationship, simmering tensions and historical conflict. Siblings have their own lives, but they can't escape that link."

Guilt begins with a bang: quite literally. In the opening moments of the first episode, Sives, who plays Jake, and Bonnar as older brother Max, accidentally run over and kill an old man on a dark road. After making the panicked decision to cover their tracks, they speed off from the scene.

With their fates now irrevocably intertwined, the chalk-and-cheese duo must work together to bury any evidence of their ill-conceived deed. Well, that's the idea. The reality – this being a pacy TV drama, after all – is far more complicated as things unravel faster than they can tie up loose ends.

"I thought a lot about how to write about siblings and what kinds of pressure you can put them under," says Forsyth. "I came up with the idea for the opening scene and wrote it out from there."

Forsyth, 41, is the man who breathed life into incorrigible comic creation Bob Servant – first as a book, then a BBC Radio Scotland adaptation and latterly a BBC4 sitcom with Brian Cox in the titular role.

More recently, he brought comedy writer Eddie Braben out from behind the curtain in Eric, Ernie and Me. The TV film, starring Stephen Tompkinson, Neil Maskell and Mark Bonnar (yes, him again), shone a spotlight on Braben's working relationship with Morecambe and Wise.

Guilt – which Forsyth proudly reminds me is his first drama series – is a marked departure from previous projects. He credits telly fare in the ilk of black comedy-crime drama Fargo as helping stoke the coals of imagination for Guilt's twisting plot with its strong vein of dark humour.

"I like shows where normal people are thrown into extreme events," he says. "I'm always interested in true stories about people who are going about their everyday lives and find themselves in extreme situations and how they react to it.

"I found it a liberating show to write because I started from that opening scene and didn't know what was going to happen next. I am usually a fairly structured writer, so it was exciting and certainly brought its challenges when I got to the final episode."

I'm curious how Forsyth treads the line between death and black humour. "No one is ever making what you would consider a joke," he says. "Humour is part of the voice I write in. I always find it very dispiriting if you watch a drama where no one says anything remotely amusing over six hours.

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"I think that starts to become unrealistic in these very weighty, relentlessly bleak dramas. Humour is one of the main ways that human beings react to pressure. It is something people use as a weapon against each other when they are angry. I find it strange how many television dramatists don't give themselves that tool in their armoury.

"Weaving humour into all the other human emotions that you want to show in a drama makes it feel richer and more real. My guide is always that, if there is a comedic moment in a scene, it's there to reflect the emotion of the character, rather than there to try and provide comedy."

Forsyth is delighted Bonnar and Sives were cast as the leads in Guilt. The pair, who between them have amassed roles in Catastrophe, Line of Duty and Shetland (Bonnar) and Chernobyl, Frontier, Game of Thrones (Sives), have been friends since they met as 11-year-olds at Leith Academy.

"They are absolutely brilliant," says Forsyth. "Both are such fantastic actors and I love their relationship. They went to school together and you really felt that history between Mark and Jamie.

"There was even some lovely little, almost sibling, tensions between them. Like I noticed the fact they both turned up looking pretty buff for rehearsal; I think they had both been slightly frantically working out ahead of meeting an old school friend.

"Little elements like that were good in terms of having a sibling-type relationship between them. They are so fond of each other and that gave a richness and confidence to the relationship."

It would be impossible to chat with Forsyth and not ask about Bob Servant. While it's been a few years since the Broughty Ferry burger don last graced our television screens, he lives on in social media, amassing a loyal Twitter following for his frank and forthright narrative on life.

Whenever the world feels gloomy, you can rely on Servant to impart his trademark gems and jaw-dropping pearls of wisdom, be it him Googling "Kirsty Wark dressed as a come-hither lollipop lady" or laying claim to giving 1980s band The Bangles their hit single Walk Like an Egyptian.

Not to forget extolling the merits of a mug of room temperature Midori or sharing his half-baked publicity stunts and glowing obituaries to tinpot dictators and tyrants.

Forsyth laughs heartily when asked whether he envisaged the bold Bob would garner such a far-reaching fanbase. "I never expected Bob Servant to have done anything, so it has been a constant source of surprise. It is a good outlet for my madder thoughts and my one social media indulgence.

"I'm not on Facebook or Instagram. My own Twitter account is largely tumbleweed, but I do enjoy Bob's account. I keep meaning to delete it. But then [the actor] Michael McKean followed him, and I thought: 'Well, I can't do that now …'"

We reminisce about some of Servant's most memorable tweets. "I like the regular obituary I offer; the last one was about Robert Mugabe," says Forsyth. "It is based around Bob knowing the deceased largely from an incident in an Arbroath roller disco in the 1970s where [a rubbernecking divorcee] banjoed themselves into pillar.

"I greatly enjoy that and get a twinge of excitement when I see the death of a suitable …" he breaks off laughing. "That's the great thing about character comedy. You can fire out a glowing tribute to Pol Pot from Bob Servant and barely lift an eyebrow. I am very fond of Bob."

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The seed for the character was planted many years ago when a then-teenage Forsyth worked in a pub in his hometown of Broughty Ferry. "He was based on these older men at the bar who were very blinkered by their own high levels of self-regard," he recalls.

"There was a guy who worked for Dundee City Council, painting the lines in the middle of the road. He had gone on a pilgrimage to Rome with his wife and when he came back, I asked: 'How was your trip?' He said it was amazing, that he had seen the Vatican and so on.

"Then he said: 'The Sistine Chapel, I went in there and that feeling in the chapel, it was incredible. And I could particularly appreciate it, Neil, being in the same line of work myself.'" Forsyth is laughing again. "That is definitely a story that could have come out of Bob Servant's lips."

Does Forsyth have any plans for more books or even short sketches? "Ach, I don't know," he says. "Twitter is a pretty good medium for Bob to be honest. I think for now he might just be resting on his considerable laurels in Broughty Ferry."

The middle of three children, Forsyth grew up in Broughty Ferry. His father and mother, now both retired, worked as a doctor and social worker respectively. "My two windows to the wider world were Broughty Ferry Library and Groucho's record shop in Dundee," he says.

"Groucho's not only through the music, but they also had this huge collection of football fanzines and that is what I first wrote for, a Dundee United fanzine when I was around 14. I used to help sell copies outside the ground. Certainly, at school, writing was the subject I was better in."

Forsyth moved to Edinburgh to go to university and around the same time began his comedy education in earnest. "The Office and Alan Partridge were kind of like The Beatles and The Rolling Stones for me and my friends," he says.

"I lived in a shared flat and we had the Alan Partridge: Live video and somehow got hold of the first series of The Office. I remember watching them over and over again, then quoting them to friends.

"When I was younger, though, it was about books and I read constantly. Roddy Doyle, Martin Amis and Kingsley Amis, the Flashman books were a big influence, as was Irvine Welsh. I remember reading Trainspotting for the first time and that was a huge influence on me.

"Here was this book that was going into areas of society that you never expected it to. All the harder boys I knew, the football casuals and the ones who went down to the Hacienda in Manchester at the weekend, started talking about this book – the humour in it and the strength of the voice."

In recent years, Forsyth has swapped city life – he has variously been based in Edinburgh, London and New York – for the English countryside with his wife and child.

"I have a very quiet life these days," he says. "I live in a rural setting. I go to London maybe every two or three weeks and pile up a day of meetings. I sometimes have to go to Los Angeles every few months, but other than that, I don't move far.

"I'm still clinging on to an amateur football career and polishing my boots on a Saturday morning at the age of 41. That is probably my main achievement. I play for the village team. I try and watch Dundee United on a Saturday afternoon through the internet.

"I lived in London for a long time and New York for a period, and part of the reason to move out to a more rural setting was to have a bit more space. We got a bit of land and I enjoy working away on that building things.

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"I never ever thought this is where I would wind up. But I greatly enjoy it. It is definitely a slower pace of life and I think very productive for me on the writing side."

What kind of things does he make when outside pottering? Forsyth breaks into embarrassed laughter. "Ah, I'm slightly exaggerating my achievements by saying 'building'. Probably building is a stretch.

"We have a little bit of land and largely what I do is ask the neighbouring farmer questions and he looks at me in utter bewilderment. That would be my main interaction with the countryside."

He is rarely idle and commissions, particularly for pilot season in the US, provide a steady source of income. "I have been writing American scripts for six or seven years now. Nothing has ever been made but it is good work and I maybe head over every six months and do the rounds of the studios, pitch ideas, try to pick up commissions, then come back and write them.

"It can be a bit of a poisoned chalice, the American telly work. I had a bit of a creative crisis about four years ago. I had a period after Bob Servant where I found it pretty straightforward getting script commissions to write sitcoms, here and also in America.

"The American work I was doing then was for the bigger broadcast networks and it was pretty lucrative, but I found it increasingly creatively quite dispiriting. I was drifting from what I really wanted to write and taking on the work for other motivations."

Matters came to a head as Forsyth sat in his office on Boxing Day four years ago, trying to finish an overdue script. "It was a studio sitcom about a multi-generational American family and I remember thinking: 'This is so far from what I wanted to be writing'.

"It was a bit of an epiphany for me. That January, I took time off from writing and I spent it reading books, researching, watching films and thinking about what I really wanted to do."

He read Braben's autobiography and that led to Eric, Ernie and Me. A biography of Irish playwright Samuel Beckett sparked the idea for Waiting For Andre, a one-off drama commissioned for Sky Arts about the friendship between Beckett and a teenage Andre the Giant, the French wrestler and actor.

"I read a book about Hollywood and there was a throwaway line about the story that became Elizabeth, Michael and Marlon which is another of my Sky films," says Forsyth. "In fact, it was that month I first came up with the Guilt idea.

"It was a creatively fruitful month. I really reset after that and from then on, I like to think I have only written things that I've 100 per cent believed in, the stories I wanted to tell and characters I wanted to spend time with. It has made me a better writer."

Seeing Guilt being filmed this year was a special moment. "There was a lovely night on the top of Calton Hill where we were filming a scene between the brothers and Edinburgh was looking beautiful in the sunset," says Forsyth.

"I could look down and see the flat in Leith I lived in when I was 30 and working in a nightclub. I could see the route I took to the nightclub. I thought back to how impossible it would have seemed to me at that age to have been up Calton Hill making a BBC drama series.

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"I feel incredibly grateful. I feel grateful for all the television I have made because it is so hard to get a show made and competitive. Every show involves multiple people placing their faith and confidence and money in you.

"Certainly, with Guilt, it was by far the biggest budget I had worked with and obviously my first drama series. I felt very grateful that night on top of Calton Hill watching it being filmed."

Guilt begins on BBC Scotland, October 24, 10pm, and will air soon on BBC2. Thanks to the Everyman Cinema Glasgow (