Pablo Grillo: "Probably the most challenging creature was the Obscurus, although whether it's a creature or not is a difficult question. It's a sort of beast that's the embodiment of the twisted and repressed magic of Credence.

"It's one of those things where it's described as a black mass, a dark force, and it's very ambiguous. An ambiguous form, something shape-shifting, which could be any size but had to ultimately express inner rage, his fury, his confusion. To be able to express those things through such an ambiguous mass is a great challenge – it becomes a piece of expressionistic art rather than a conventional approach to animal psychology which you can physically anchor.

"It's very interesting and very challenging on a technical level as well as a creative one because you're dealing with a lot of simulations and FX that have to be controlled to be expressive, but are unruly because simulations do their own thing. So finding the right result, the right layers to use to get that right is a big old challenge."