Isaacs, who grew up in Liverpool and fell into acting while studying law at Bristol, has racked up at least 30 Hollywood movies as well as several high-profile TV roles and a smattering of stage roles.
Although he has never exercised much control over his parts, he's certainly played his fair share of violent men.
He's been an RUC officer struggling to throw off the legacy of the Troubles in Gary Mitchell's play The Force Of Change; ruthless gangster Michael Caffee in the recent American TV series Brotherhood; and a working-class thug (Chris) in last year's searing docudrama Scars on Channel 4. Isaacs points out he has also played priests (in The End Of The Affair) and transvestites (in Sweet November).
Have his tougher roles, playing hard-boiled mobsters such as Caffee and Chris, helped feed into his understanding of the threatening world of Pinter's play? And have they given him insight into Ben and Gus: two assassins who, on one level, demand the sympathy of the audience?
'I've certainly met plenty of soldiers and criminals in my time, men who've killed people, and they've become this swirling soup of research material from which I can take bits in order to make the character I'm playing work,' he says.
'And most of the violent characters I've played have come from very hard, impoverished backgrounds. But everything that happens to somebody happens for a specific reason.
'With Pinter, the joy is in the mystery. If there's no mystery, then it becomes a crap performance.
'This play is about a relationship between two men and there's some part of Harold's world view in every single moment of it and yet it gives us no conclusion.
'Great storytellers such as Harold stick the story out there but if you ask them to condense it into a few sentences, they look at you in horror because if they could condense it, they would.'
Given the extent to which he researches his characters, it's no surprise Isaacs enjoys playing Potter baddie Malfoy.
'I'm always so tortured about work - it's always about trying to find the truth in a character - so it's great with Harry Potter to devote my attention to devising ever-more ingenious ways of blasting Gary Oldman with a wand,' he says.
'That said, I'm not in the sixth film because Malfoy is in prison. There is no one on Earth waiting more eagerly than me for JK Rowling to publish the seventh book this summer. I'm desperate to know if I'm in it.'