Harry Potter author JK Rowling spoke exclusively to Newsround's Lizo Mzimba.
Here's what she said.
Lizo: What was going through your head as you watched the film, and how did you feel?
JK: I'd say a week from seeing the film I was very excited. And the closer the viewing came, the more frightened I became.
To the point that when I actually sat down to see the film, I was terrified. Because it was way too late if bits were wrong - if Snape hadn't got long blond hair or whatever.
But at the end of the film I was happy.
Lizo: That's good, because so often writers say they don't enjoy their books being made into films.
JK: Chris Columbus, the director, made me two promises. That it would be an all-British cast, and that he would keep it as true to the book as was possible within the constraints of film.
And he's kept both promises. I also thought the three children, Harry, Ron and Hermione, played by Daniel, Rupert and Emma, were superb. I was very pleased.
Lizo: It's been reported that you talked about the "back story" of some of the characters with some of the actors. Is that true?
JK: I'm very secretive about these things, but yes I did. I gave certain people certain information. I gave Robbie Coltrane some background on Hagrid, which I think he used superbly, because I think he's an absolutely fantastic Hagrid.
I also gave Alan Rickman a little bit on Snape.
Lizo: Quidditch is something that's been in your head for years and years, and now you've seen it on screen. What was your reaction?
JK: I loved it. I loved the Quidditch match. It really looks the way I wanted it to look. They asked me ages ago, what is Quidditch, what does it look like? I always said it's violent and scary. But I couldn't imagine how they were going to do it, but they have, and it looks so real.
I was looking forward to that for the whole first half of the film, and it really lived up to my expectations.
Lizo: What do you think fans of the books are going to think of the film?
JK: I hope they like it, I really do. I think the fans of the book will be happy. There is an awful lot of my book up there. All the important bits, I'd say.
It's my plot, and I think it's a very faithful adaptation. And there are spectacular performances: a really wonderful Hagrid, McGonagall and Snape, and a fabulous mountain troll - that's one of my favourite bits!
I really hope the fans are happy.