"You can't get away with anything there, you get joshed and played with. You can't have any airs or graces."

David insists he's been incredibly lucky to get such grounded young stars for Harry Potter.

"You cast for the moment, you have no sense of how someone is going to survive or how they're going to develop. I've been very fortunate with the kids. There are so many cases of young kids being damaged by the experience or going off the rails and fortunately with Dan, Rupert and Emma that hasn't happened to them... yet!"

He continued: "I don't think its going to happen to them, they're pretty together, really good kids, well - young adults now."

David revealed work is about to begin in the new year on the seventh movie in the series.

"We finished the sixth film which'll be coming out next summer and we start filming the seventh which we'll be breaking into two parts for a 54 week shoot starting in February."