J.K. Rowling: What has been phenomenally enjoyable, has also been the biggest challenge, which is that - in contrast to the first movie, which was a relatively straightforward narrative (although a lot was hidden), this is a complex story. So that was simultaneously very challenging to write and enormously satisfying to write. I do really enjoy a challenge. It was finding the structure to make sense of all these different strands, so that people understood when they hit our climactic scene, what was at stake, and why characters were reacting as they were reacting to Grindelwald.

You've also got this thread running through the movie, which is: Who is Credence?

The Crimes of Grindelwald have historically been crimes of violence and terror. However, as we meet him in this movie - we do see him kill, again - we see him kill lightly, cold-bloodedly. But we also see his immense seductive power.

And it should be shocking - I wrote it and it was shocking to me - he's a sociopath. We watch him corrupt an innocent, and we see his immense, duplicitous gift of speech, in the final scene where we really see the danger of the man.

The appeal of moving on from the UK is two-fold. Firstly, for the sheer sensory pleasure of looking at wizards in different cities and habitats. It's so visually interesting to mix up the architecture and the flora and fauna of different of different continents. So, yes, there are definitely different places I would like to go. It's very freeing not to be stuck at a school. So in a very prosaic level, because it's so much fun to press into to different places, and we've got witches and wizards there.

But, also, it really serves our plot. With the Potter movies, one had the sense of Voldemort's threat to the whole wizarding world, you never saw how it was affecting people and the rest of the world. Well, this time, I wanted to look at a world war, a world uprising, and actually go see that. Not just stay with one character in the UK and see what's happening.

You want to do new things, you want to do it differently.

Just looking at movie two, moving to a new city where I felt there would be a much more fluid relationship the magic and the mundane. In the 1920s, it had to be Paris - of course it had to be Paris! Paris was genuinely, at that time, in reality, an place where you had a far more bohemian society. You have people of varying backgrounds going into Paris and living this more bohemian life. The artistic community was far more diverse in every sense, than you would've met in other cities in the 1920s. So it felt perfect to go here.

The Dumbledore we meet here is clever, he's gifted, and he's very mysterious. No one really understands why this man is a school teacher. He appeared to be born to be the preeminent in the wizarding world, he appeared to be head of the International Confederation, to embrace a role that would make him part of wizaridng history. But he disappeared to this school in Scotland and stayed there. Why?

In this story, particularly, the battle the light and dark is a very interesting way of phrasing of what's going on in this movie, because it is precisely our human drive to simplify and divide everyone into black hats and white hats. That has resulted in a huge destabilization in the wizarding world.

Grindelwald inspires an almost cult-like devotion in his followers. It's very easy, as an outsider, to say 'bad'. We have a tendency to simplify; if we look at what he's saying and analyze it, it does fall apart. However, if you're not paying a lot of attention to the substance, and the inherent contradictions in what he's saying, it sounds very seductive, very plausible, and can persuade people that you or I might consider good people.

When I met Jude [Law], we had this very intense meeting, just the two of us, where I basically told him everything about Dumbledore that I've ever known, but not told anyone else - literally, I've never told anyone some of the things I told Jude. He took notes and asked questions, and yeah, it was great. He's done an amazing job. I really like his performance, I think it's going to surprise people. There's a lot more to go for his character, so I think people will be rightly hungry for more.