"They started in February, but they go until the end of the year because they broke for some of the young actors to take their summer exams," explains Isaacs, who had to postpone his part in order to complete a British TV series.

"My bit is in September and October. It's a big blue-screen battle for a lot of it so that will be a laugh," he says. He adds, "We always have to sign these incredible confidentiality clauses where we offer up our children's eyeballs if we ever break them. I've never quite understood since they're the most popular books on the planet, but anyway, I think I'm involved in the big battle."

Isaacs admits, "I torture myself by always trying to be interesting and human on screen, but every now and again it's great to be in 'Harry Potter' and just rip it up and be as singularly evil and unpleasant as possible."

Right now, Isaacs is gearing up for the July 9 premiere of his new Showtime series, "Brotherhood" — in which he plays a gangster from Rhode Island whose brother (Jason Clarke) is a local politico. "It was deliciously enjoyable to get my mouth around the very peculiar Providence accent," he claims. "For a British actor, it's a real godsend having a different voice like that. We're fascists about voices in England. As soon as somebody opens their mouth, everyone can tell not only where they come from and what kind of education they have, but what they aspire to do in life. For this part, once you start speaking like the character, you start thinking like him."