"I saw the wedding (Diana's 1981 wedding to Charles, Prince of Wales) and everything and she is a very inspiring woman and it's kind of impressive how she sort of changed the whole gearing of the tradition of the royal family and opened it up and brought warmth to it."
Hirschbiegel added: "I am delighted to have such a truly exceptional actress who embodies the warmth, humanity and empathy of such a global icon as Princess Diana."
Khan will be played by Lost actor Naveen Andrews, who recently praised Watts as perfectly cast for the role.
"I don't think anyone else could have done it - there's no one else," Andrews said.
He added: "Whenever you work, usually if something has a chance or the potential to be good it usually starts with a good script and I'm very happy with the script and Naomi is doing it and she's wonderful ... I'm very much looking forward to shooting."
On Tuesday, Watts was snapped stepping out on the set of the movie in London, wearing replicas of outfits worn by the Princess, and sporting Diana's trademark mid-90s haircut.
Watts donned a black blazer, pencil skirt and heels while filming one scene.
She was later spotted in an evening dress and with immaculately coiffed short blonde hair, while filming a scene outside the Royal Opera House.
A right royal double take? Decide for yourself.
Check out snaps of Watts as Diana here, here and here.
Caught In Flight will be released in cinemas next year.
- Blogger Bites Back