"The Harry Potter world is a crowded world, and we always try to choose actors who are very vivid," says casting director Fiona Weir, whose first "Potter" film was 2005's "Harry Potter and the Goblet of Fire." "Even though they can play into the heightened 'Harry Potter' world, it's important they remain truthful and naturalistic."

"Helena [Bonham Carter] is such a lovely, highly educated lady," says Felton. "She'll be telling me about something interesting that happened in the 16th century and straight away, when they roll cameras, she turns into the most evil woman I've ever seen in my life."

"It's a unique phenomenon," she explains. "Some of the cast came to it knowing the minutiae of the stories, and for some it's their first experience with it and they devour the books. More than one actor has said, 'My children will forgive me for everything I ever do if I get to be in the film.'"

One of the toughest roles to fill was Lord Voldemort, a villain as mythical as Darth Vader and Hannibal Lecter combined.

"That was a big challenge, but Ralph was the only person, really, for that part," says Weir. "I'll never forget him in 'Schindler's List.' There was no one in the world more frightening than the commandant he played."

Bonham Carter was also selected based upon her work in a completely different genre.

"Ever since 'Fight Club,' she's been playing quite frightening people," says Weir.