The idea came from the studio … We said, “Let’s see what we can do.” We began to work on it with IMAX. They’re very good partners, they’ve done a lot of 3D for IMAX – this is very different. But they were great partners. We looked at in all around 250 shots. It was interesting. Scenes that I thought would not work actually worked quite beautifully. For example, the dance worked really well. It wasn’t shot with longer lenses.

This film actually had a lot more long lens shots than the next film, and long lenses rarely look good with 3D. You’re compressing and yet you’re expanding – it’s odd. So the dancing actually went really beautifully. You highlighted their loneliness within this tent. We didn’t make the tent massive but you were able to isolate them, and that was really beautiful. The way to approach 3D, and the way we did it when we were working on it, was as a character. How do you enhance the atmosphere, enhance the storytelling.

What happened on HP7, Part 1 was we realized we didn’t’ have enough time to do it right. We could’ve done it, but it would’ve been crap. No one wants that. Why do it now? Why screw up now? The studio were pushing, but when we got to the point where it became clear that we weren’t going to be able to achieve the quality that we wanted we had a very adult conversation with them and said, “Look , this is a problem.” And they heard us. They supported us. And we abandoned it.

Part 2 is different. We’ve got more time. We’ve begun the process already. It’s also a very different film. It’s ore of an epic film – a lot more wide angle lenses. And 3D works best with that.

We shifted where it ends. We had an initial discussion, this is where we ended it, where it was in the film. Then we revised that because we were concerned. Something happened at the end which was similar to what happened in 6 and 5: the loss of a character – the death of a character. So we then moved it back where we had it earlier in the film.

But then when it came time to cutting it, we put it back to the original thought – not where it is in the script, but where we originally thought about it. And David added this additional scene of Voldemort getting the Elder wand. Because it gave a sense of emotional closure. Or the completion of something emotional. And then with the wand you had a sense of, "Ah, this is what the challenge is. This is what we're going to in the next film.”

Stuart Craig is always refining Hogwarts. If you look at the Hogwarts from the first film and look at Hogwarts now, it’s a very different beast. And the Whomping Willow that was in the first film has moved around a lot certainly over the first few films. the first film, in terms of visual effects, the quality wasn’t as good as in subsequent films. That industry, and what we can do, has changed immeasurably over the ten years that we made the films.

On the first film I think we were too rigid in our use of visual effects. We let the visual effects lead us as opposed to using the visual effects to work for you. Ultimately, visual effects is not as mysterious as it initially appears, or appeared to us Actually, nowadays, you can do so much but it has to all be about the characters. You’ve got to serve your characters first, and that’s when visual effects is at its most successful. So those were the mistakes we had early on.