“But then, as the 30s go on, you see how the changes alter their friendship and the indignity and the horror that Maurice suffers. I found that very moving and very engaging, and somehow the specifics of one man’s life made it more possible to imagine it happening multiplied by six million.”

When the go-ahead finally came to start shooting Good, Isaacs was still committed to another project, the American gangster series Brotherhood.

He was determined, though, that the opportunity wouldn’t pass him by.

“The network were amazing because they knew there was this thing I cared so much about, so we shot all my episodes in the wrong order so I could have a big break,” he says. “I ended up doing Brotherhood in Rhode Island one week, then racing to the airport, getting off the plane and being in 1935 Germany.”

For the 45-year-old actor, who proudly describes himself as “a 230th generation Jew”, Good is a very personal project.

A former King David pupil, his great-grandparents came to Liverpool from Eastern Europe, and he was last in the city for Holocaust Memorial Day, in January, 2008.

He has lived in London since the age of 11, but still has ties here, not least through his beloved Liverpool Football Club.

Although, he admits, he has had no luck convincing his two daughters to follow that particular passion.

“Oh, they won’t even kick a ball,” he smiles. “They’re not interested in football at all. They keep pretending because they know dad’s a sports nut, but I know really they’re just placating me.”

Lily, who’s seven, and three-year-old Ruby play a pivotal part in Isaacs’s life, along with his wife Emma, a documentary film-maker.

He is very much a hands-on, rough-and-tumble kind of dad, he says, and he’s been fortunate that his work to date hasn’t meant long family absences.

“My kids have been lucky because they’ve lived in America, Australia, Canada and all kinds of other places,” he says. “And for the last season of Brotherhood we all went to Rhode Island together, although my oldest is at school now, so for the first time I’ll be up against some choices.

“I was offered a lot of American TV series this year, and if I’d been available I would have had to decide whether to fly backwards and forwards or to move the family out there. It’s tough, because until now we’ve always been able to be together.”

A conspiracy of schedules has actually seen him spend much of this year at home, although in the summer he’ll begin six months devoted to the newest Harry Potter film. Lucius Malfoy, he accepts, will entirely take over the latter half of his year.

Before then, he has Good to promote and the thriller Green Zone, with Matt Damon and Greg Kinnear. And there’s the little matter of a Bafta nomination, too, for his role as Harry H Corbett in the BBC4 drama, The Curse of Steptoe.

It’s only his second award nod, he says. “And with the previous one, a Golden Globe for The State Within, the ceremony was cancelled because of the writers’ strike in America,” he smiles.

“I’ve presented at the Baftas three times and at the Empire Awards. and I like it because I like making myself nervous,” he says. “As an actor you don’t really get nervous; if you get something wrong you just do it again, but if you write yourself a little gag there’s something rather exciting and terrifying about telling it to an audience of your peers and half the country watching on telly.