"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."