Jason Isaacs walks in the door carrying a big fat stick with a snake's head on the end, and already it's clear he's going to bring a whole new brand of creepiness to the magical world of Hogwarts.

    don't need to see him in his costume, swathed in ermine and paisley, clouds of blond hair falling over his shoulders, looking for all the world like the actor Julian Sands. He doesn't need to mention that he has based his voice for Lucius Malfoy on 'one particularly noxious English art critic'. It's there in the way he twirls the thick wooden handle between his fingers, raps it on the table, swings it menacingly. It's there, also, in the knowledge that Isaacs excels at baddies: his Colonel Tavington in The Patriot had the UK press booing and hissing at the audacity of Hollywood in making its British characters so evil.

The stick, he explains, is a star in its own right. 'I said to Chris Columbus: 'I thought I'd have a stick.' Chris said: 'What? He's got a walking disorder?' I said: 'No, it's a kind of cropped thing with a snake's head.' He not only let me go with it; he let it steal every scene I'm in. I'm poking people. I'm hooking them. I'm rapping Draco about the head and the knuckles. In fact, the very first scene I was in I had to flounce out the door. Chris said: 'Don't forget to close the door behind you.' I went: 'Er, yeah, or I could just wave my stick and the door slams.' He said: 'Okay.' I thought: 'This film is going to be fun.''

It's often the actors who play the foulest villains who are the nicest. Isaacs is like that, decent, funny and self-deprecating, the kind of guy who lets people in front of him in queues, who has 'brought a disproportionate number of kids to visit the set', and who ended up in acting just because he was offered a place at drama school and didn't know how to say no. 'In real life,' he has said, 'I am a cringing, neurotic Jewish mess.'

Yet still there's something that allows him to morph into the most cringingly unspeakable characters, something that prompted Daniel Radcliffe to suggest that he's even slimier than Alan Rickman's Snape -- 'really scary; really creepy'. And it's something that leads him, however much he might resist (after The Patriot he turned down a string of lucrative bad guys) constantly back to the dark side.

His next role, for instance, is as Captain Hook in PJ Hogan's new version of Peter Pan, about to start shooting in Australia. 'It wasn't part of my plan to play two of the greatest villains in children's literature back to back,' he says. 'That would be rather suicidal.'

For all he takes Harry Potter for what it is -- a riotous bit of fun -- he is meticulous about his role, seeking out the motivation behind the pantomime. When the costume designer suggested pin-striped suits, Isaacs came back with fur and flowing locks. 'There's no point being in a film about wizards and dressing like a businessman,' he remarks.

Meanwhile, he found inspiration for Malfoy's character in the 'people getting elected all over Europe, talking about immigration'.