'It was mass hysteria': Jason Isaacs on groupies, theatre bores and snogging James Bond

To millions, Jason Isaacs is one of Harry Potter's arch enemies – but his wife prefers him as a Scottish TV detective.

Lucius Malfoy never got to keep his 'Paris Hilton wig'. Or rather Jason Isaacs wasn't allowed to take home Malfoy's so-named, blond hairpiece when they finally wrapped the last Harry Potter movie.

"I won't name the cast member, but he blew it for everyone by nicking all the gold coins from Gringotts Bank on the first film," says Isaacs, who impersonated JK Rowling's Death Eater over the entire franchise. "After that there was a Fort Knox lockdown of all the props, although they did give me my wand… my wand in a cane… because it was my own idea."

He delivers this anecdote – in fact the entire interview – in parenthesis-heavy hyper-drive, something to do with his jet-lag, perhaps, or an afflicted sun in Gemini (he does have a June birthday). It's not quite the oh-so-brainy stream-of-consciousness of a Stephen Fry or a Benedict Cumberbatch, more a restless loquaciousness, and Isaacs is disarming enough to say at one point: "I talk utter bollocks… I believe everything I say in the minute".

He's difficult to have about the house, he admits, and having relocated his family to Los Angeles while he filmed the (now cancelled) high-concept US TV drama Awake, and having decided to stay there because his two daughters, Lily and Rose, were thriving at an American school, his wife persuaded Isaacs to go away to Scotland to film a new series of BBC1's Case Histories. Perhaps he was getting under her feet.

"I'm a big baby," he admits. "I have a very short, intensive attention span… I cause all kinds of domestic mayhem when I'm kicking around at home. Anyway, having made an absolute decision that I was never going to be separated from my family ever again, my wife insisted that I come back to Scotland to make it, because Case Histories is her favourite show."

Her favourite show and her favourite alter-ego, it would seem, for his wife, the documentary filmmaker Emma Hewitt, is just a bit in love with Case Histories' soulful, damaged Edinburgh gumshoe, Jackson Brodie. All a bit confusing for a chap. "I watched the first episode last night because I'm still jet-lagged and couldn't sleep," says Isaacs. "My missus was going to bed and said 'Darling I can't watch it… I'm knackered'. So I put it on and she sat there glued for an hour and a half, and she kind of shook her head and sighed disappointedly at me and went 'God I love Jackson Brodie'. It was the most back-handed compliment-stroke-insult that I've ever had."

Based on Kate Atkinson's novels, Case Histories is a pleasingly relaxed and characterful addition to the over-populated world of television detective drama. The first series was screened in 2011 to favourable if not ecstatic reviews (I liked it a lot), and a new batch of three feature-length episodes begins with an adaptation of Atkinson's book, Started Early, Took My Dog, which guest-stars Victoria Wood and a border terrier whose real name is Crunchie.

"God, that dog was a fucking nightmare," says Isaacs. "Kate wrote it in that this dog was following me everywhere and jumping up in my lap, but it wouldn't do any of those things. Crunchie would only respond to sausages, so I not only have sausages in my hands and pockets, I have pâté smeared all down my leg."

Never work with children or animals, although Isaacs' criterion for whether or not he should take a role is whether he could stand seeing another actor in it – or what he describes as, "The sure knowledge that if somebody else did it I would be bitter and resentful for a long time". It's just as well that he has criteria, because Isaacs says he is hopelessly indecisive.

"It reminds me of the time before we had kids and the wife and I would leave the house in West Hampstead with no limit where we could go to dinner, drive around, cruise past a dozen restaurants, stop, look at menus and end up at home with scrambled eggs. I'm about that decisive about work."

This does bring an element of the unexpected to his oeuvre, and upcoming projects include a cowboy thriller, Sweetwater, co-starring January Jones – Betty Draper from Mad Men – and an episode of a YouTube web drama called Kendra. "I like to have variety," he says.

Born in Liverpool in 1963, Isaacs was raised in a tight-knit Jewish community in the suburb of Childwall ("Since the only reference point people have is The Beatles, that was round the corner from John [Lennon]'s auntie," he explains helpfully). His father, a jewellery-maker, moved the family to London when Isaacs was 11, and those years coincided with the rise of the National Front.

"There were constantly people beating us up or smashing windows," he says. "If you were ever, say, on a Jewish holiday, identifiably Jewish, there was lots of violence around. But particularly when I was 16, in 1979, the National Front were really taking hold, there were leaflets at school, and Sieg Heiling and people goose-stepping down the road and coming after us."

School was Haberdashers' Aske's in Elstree, Hertfordshire, an independent boys' school where his contemporaries included David Baddiel and the film critic Mark Kermode (Sacha Baron Cohen and Matt Lucas are also alumni). "I have very un-fond memories of my school life. It was only when I got to university and drunkenly stumbled into a theatre audition that I found what I wanted to do," he says.

With Isaacs safely at Bristol University, along with his brothers (who became, respectively, a doctor, a lawyer and an accountant), his parents emigrated to Israel, partly due to the rise f of the NF. "That might have been what galvanised my parents to get out," he says. "My dad got into a lot of scraps in the 1930s when he was a young Communist. I'm always trying to persuade them to come back, because this is home to me. But they felt so uncomfortable as young people and were so defined by their experience being Jewish during the 1930s and Forties."

But back to Bristol University in the early-1980s: Isaacs' drunken audition led to a full-blown love affair with acting; he went on to appear in over 30 undergraduate plays before landing a place at London's Central School of Speech and Drama (which is where he met Emma).

Leading television roles (in ITV's yuppie saga Capital City and Lynda La Plante's 1992 BBC drama about ex-paratroopers, Civvies) followed, as well as playing the gay Jewish office temp, Louis Ironson, in Tony Kushner's Angels in America at the National Theatre, and getting to kiss Daniel Craig every night. "I've played a lot of gay parts, and it's a barrier for me to get over snogging men," he says. "But Daniel was so easy… he's such a sexy man."

Generally, however, the stage would seem to hold little appeal. "Most theatre bores the living shit out of everybody who sits in front of it," is his rather startling assessment. "But because most of the people in the audience go twice a year they think it's their fault, and it's a bitter cultural pill that's good for them."

Fortunately, he has been kept busy elsewhere – on television being Golden Globe-nominated for his British ambassador in the political thriller, The State Within, and Bafta-nominated for his Harry H Corbett in the BBC4 biopic The Curse of Steptoe. On the big screen, a huge variety of roles that tended to the villainous (the common fate of the English actor in Hollywood) have included the priest in Neil Jordan's 1999 adaptation of Graham Greene's The End of the Affair, a sadistic British officer in the Mel Gibson American War of Independence costumer, The Patriot, and a US Ranger in Black Hawk Down.

But these have inevitably been dwarfed by his impersonation of Lucius Malfoy in all seven of the Harry Potter movies. This is the point in the interview when he tells me that he talks "utter bollocks", because earlier Isaacs had been explaining that he prefers anonymity to fame. "Completely repugnant," is how he described the prospect of celebrity. And now, well…

"I didn't get to do the publicity tours until I did the last one and I got to realise what I'd missed out on because everywhere you went in the world there was this mass hysteria," he says. "Mass hysteria for the minor characters like myself – I can't imagine what it would be like for Daniel [Radcliffe] or Emma [Watson] turning up somewhere… there'd be people spontaneously combusting.

"It hasn't abated at all… not even slightly. I think it's because new generations keep reading the books and then are drawn to watch the films. My own daughter wanted to come to the last Harry Potter premiere and I said 'Darling, I think they're a bit grown-up for you' – she was nine at the time – 'and I'd rather you read the books before you see the films'. And I was away shooting something for about a month and she read all seven books in a month. She wasn't much of a reader before that."

A rather more obscure but thoroughly post-modern type of fame is the meme, 'Hello to Jason Isaacs', that has resulted from a regular shout-out by his school contemporary Mark Kermode, on Kermode and Simon Mayo's Radio Five Live Film Review. "It's like a creature out there, some Dr Seuss creation running amok," says Isaacs. "I met Alan Parker once and said 'Hello', and he said 'As in "Hello to Jason Isaacs?"'. And I was on a hike in America and I was in a canyon and I heard someone yodelling from a neighbouring canyon, 'Hello to Jason Isaacs'." In fact, they had to re-shoot some of the crowd scenes in Case Histories because there was someone standing behind him with a banner saying… Well, you know what it said.

On the less frivolous side of celebrity, Isaacs had a stalker for eight years, a woman he has regularly taken to court in order to enforce a restraining order. More benign are the regular female groupies – Isaacs' equivalent of Benedict Cumberbatch's charmingly so-called 'Cumberbitches'.

"They have followed me for decades, and turn up everywhere and anywhere I am," he says. "They're well-to-do women and have resources. I hosted a charity gala recently in South Central LA for homes for women who get out of prison – and they had bought a table. When I was on stage in the West End they flew from all over the world, including Australia, and booked tickets for every night. And they even send beautiful gifts for myself, Emma and the girls. You'd think they were my aunties – except when they're having their photograph taken with me they have their hand on my arse."

'Case Histories' begins on BBC1 tomorrow

Shot on location in one of the Bulgari Suites at the Bulgari Hotel & Residences, London, bulgarihotels.com, 020-7151 1010; grooming by Claire Portman