Here is the part of the interview in which he discusses the Potter movie:
LR: Speaking of Alfonso Cuarón, he has a huge movie coming out with Harry Potter and the Prisoner of Azkaban, are you expecting it be totally different from the first two films?
Guillermo del Toro: I already saw it.
LR: Really? How was it? And is it darker from the other the two films?
Guillermo: It’s absolutely fantastic. It’s not only a beautiful Harry Potter film, it’s an absolutely delight of a film period. Its just…Alfonso imagination for anyone who saw Little Princess, it’s extremely rich and Harry Potter and the Prisoner of Azkaban is incredibly detailed, incredibly full of spectacle and intelligence. I mean I was blown away by it, I think it’s going to be a huge success. I saw it London and it’s simply fantastic.
LR: Would you have taken a project like Harry Potter?
Guillermo: They actually offered me Harry Potter and the Prisoner of Azkaban before Alfonso. Which is actually very good in terms of realizing the way Latino filmmakers are regarded. I was very sincere with what I said before. I love the Harry Potter universe, I love the books but I felt more passion towards doing Hellboy. It was a project that I had been involved so long and if I didn’t do it now, I could never get to do it. I'm extremely happy that Alfonso took it and he did a fantastic productive job. I think it’s one the best children’s movies I have ever seen in my life.