IN the shadow of the Forth Bridge, Jason Isaacs looks at the grey water ominously. I would too if the next day I was going to have to jump in it. "No, I'm not looking forward to it," he says, as the cuts and bruises that stipple his face are touched up by two make-up artists swaddled in enormous puffa jackets. "I'll probably have to do it 20 times. It might kill me." He stomps his feet and shivers.
The heroics, and the cuts and bruises, are all down to one man: Jackson Brodie, the central character of four hugely successful novels by Edinburgh-based writer Kate Atkinson. Isaacs, best known for his recurring role as Lucius Malfoy, complete with bleached blond ponytail and cape, in the Harry Potter movies, is the man with the responsibility of embodying Brodie on the small screen.
"People have already got a version in their head of what Jackson Brodie looks like," he says, "They had an idea from the books what Lucius Malfoy looked like too but now, thankfully, they think he looks like me. With Brodie, people have known him and loved him for years, I just hope they can make the adjustment."
In Case Histories a six-part adaptation, three two-part stories, in which the Cambridge of Atkinson's novels has become the cobbled cragginess of Edinburgh, Isaacs plays the former soldier and policeman turned private investigator who has a soft spot for waifs and strays and an inability to say no to cases. For Isaacs, stepping into the crowded and cutthroat world of the TV detective might be a little nerve-racking but he couldn't be happier with the subject matter.
"It was difficult for all of us at first because we love the books to really, fully embrace how in order to honour them we had to be slightly irreverent," he says. "What makes this work is the people and the places and the relationships but not necessarily the way the story is told.
"It's not that you don't need the overarching structure of the plot but what you're watching is the people, what they think of each other and how their relationships are going to resolve. That's what all drama is about.
"It's not a competitive sport. We're not setting out to top the other detective programmes, but I just hope – and it's what I always hope when I sign up for these kinds of projects – that it stands up as good drama."
He's pretty sure that it will, if his first test audience are anything to go by.
"I'm always slightly embarrassed to say something that I'm working on is really great," he says. "But I watched the first episode with my wife, mother-in-law and grandmother-in-law and they all thought it was great. They wanted to see the rest of it, which was nice, but it hadn't been made yet."
Isaacs is a fan of Atkinson's work and he knows the Jackson Brodie novels intimately from having lent his voice to the audiobooks. "I've played all the parts," he says. "I've done all the funny voices." That was at least part of the reason Isaacs took the role of the troubled detective.
"I just felt it was fate really," he says. "I'm quite poorly read and I spend most of my life now reading newspapers and magazines and scripts, and yet these books I know intimately because I've played all the characters, all of them. I knew that if somebody else did it I'd be annoyed."
The other motivation for taking the part was a more prosaic but no less pressing. Having spent the past few years working on movies and TV projects in the US, Isaacs wanted to be at home with his family. "We had kids late," he says talking of his wife, Emma, "so I see my friends' kids and for them it's all about car keys, cash and rehab, but mine are still small enough to want to make papier mch with me and go ice skating and I want to be around for that stuff. "I came back from two or three stints in America at the start of this year and I said to my agent I have to work in England, I can't be away."
The next day Isaacs was offered a couple of television things, one was a show for ITV and it sounded really good and the other was Case Histories. But it didn't quite go as planned. Isaacs had been given the "pre-Edinburgh" scripts so it was only two hours into a meeting with the production team when it was mentioned that Edinburgh is almost a character in the series that he realised his plans to stay at home to be able to put his kids to bed hadn't quite worked out.
"But it's been a great way to spend a few months," he says. "Edinburgh is a great city and the kids love coming up here at the weekends. It's just a hop away and I get to concentrate during the week. It's all worked out rather beautifully."
In many regards, Isaacs' career has worked out rather beautifully, albeit in a low-key way. The malevolent Malfoy might be his most recognisable role, but that shouldn't be allowed to obscure a growing reputation in the US from parts in The West Wing (as a photographer who has a fling with Donna Moss) and Brotherhood as well as his astonishing turns as Harry H Corbett in BBC4's The Curse of Steptoe and as the lead in the gritty and compelling Channel4 docu-drama, Scars, in which he played a man tortured by his horrifically violent past. The thing about Isaacs is that he's one of those actors who in a way disappears into his roles. It's a tribute to his skill as a performer, but it probably doesn't help to build a public profile. Not that that seems to interest him particularly. He says that if he reads a script and he doesn't like it, or can't find anything in the story to respond to, he "can't say yes".
"I wish I could sometimes, but I can't," he shrugs. "I've avoided being the man with the gun solving the problem a lot of the time.
"I went to America for pilot season last year and I was offered lots of pilots which wasn't a surprise because The Brotherhood which I'd just been in was very successful, but they were all men with guns who solved problems. They've all now been made and they've all turned into series which are successful. The only regret I have is that the cheques aren't arriving, I never regret not doing the work, I just wish they'd send the money."
As to how he does choose a role, he smiles wryly.
"Mostly it begins with the question: is there any chance that I could be not shit? That's the first thing, can I pull this off? Then it's usually have I done this before? I don't want to get above myself and sometimes you've got to pay the bills, but that's happened to me very rarely thankfully.
"Finding interesting things to do in your life as the clock ticks away is tough as is feeling the responsibility not just to entertain yourself but to try and put decent stories out there. I've got kids. I don't want to put toxic stuff out there in the atmosphere. I don't want to make people more scared or more angry. I want to put things out there that entertain and engage and provoke."
His attitude to work is based on choosing what he wants to do and pretty much ignoring everyone else. It's something he credits to The Scotsman, he tells me with a smile. He was in his first year at university and together with some friends he was in a play at the Fringe, financed using their own money. Out of an audience of five on their opening night, two were reviewers. The Scotsman writer said the show was dreadful, the worst example of self-indulgent student bilge. Devastated they bought a bottle of tequila and went up Arthur's Seat to drown their sorrows.
"Later that day, after we'd wobbled back down, the Evening News came out with a glowing, five-star review. We put both on a poster and asked audiences to decide for themselves. We sold out all summer and I realised I've got to follow the beat of my own drum."
He says he takes a similar approach to reviews, explaining that you always remember the bad and forget the good and that he knows only too well the frustration of being in things which are critically lauded but struggle for an audience.
"I'm kind of hankering after a massively successful critical disaster because I've been in enough flops that have been well reviewed," he says, "It's very nice when critics like you, but it's more important to strike a chord with the public because feeding your ego only goes so far. "I'm hoping that Case Histories is watched by everybody and the critics despise it." He laughs.
• Case Histories is on BBC1 on tomorrow at 9pm and continues on Monday, 6 June
• Jason Isaacs was born in Liverpool, and his family moved to London when he was 11. He was a pupil at the Haberdashers' Askes' boys school. He went on to study law at Bristol University but dropped out to pursue acting as a career. He is married to Emma Hewitt, whom he met at drama college, and the couple live in London with two children, Lily, eight, and Ruby, six.
• He's been the villain alongside Mel Gibson in The Patriot, a Jewish psychoanalyst alongside Viggo Mortensen in Good and Captain Hook/Mr Darling in Peter Pan. Nominated for a Bafta for his role as Harry H Corbett in BBC4's The Curse of Steptoe, he was also nominated for a Golden Globe for his performance in US drama The State Within.
• Asked how he started acting, he said: "Oh bloody hell! I don't know. I think I was probably always a liar, I just get paid for it now."
• On how to act well: "When you see actors messing around on set and you think they are being very childish or even unprofessional what they are doing is keeping themselves loose because what we have to do is keep completely relaxed and not get bound up in the tension that everyone else is feeling."
• On awards: "I think all prizes are kind of ridiculous. I've never taken them seriously, so I'm not about to now. I really just feel like I'm piggy-backing on other people's talent and getting all the credit for it, because it's basically an award for being a lucky enough b*****d to get one of the best parts going."