Heart of the Beast director David Ayer has admitted shooting the film was a much "bigger challenge" than he had anticipated.

The adventure movie stars Brad Pitt as army veteran James, and focuses on the character's relationship with his beloved dog Odin.

And while David has helmed huge movies in the past - with complicated, epic battle sequences - it's Heart of the Beast that ended up being one of the most taxing, for surprising reasons.

He told IndieWire on their Filmmaker Toolkit podcast: "It was a bigger challenge than I understood going into it. I really didn't know what I was in for."

"The simplicity of it exposes the filmmaking in ways I never imagined. Your decisions, tone, mood, and performance count more than ever in that situation.

"There was nowhere to hide."

David signed up to direct the movie when he found himself having a huge emotional response to Cameron Alexander's script.

He explained: "I think  it triggered part of my own grief over the loss of my father when I was young, and this sort of aspirational desire in me.

"I wish I had someone who looked after me the way James looked after Odin. I wish I had that kind of unconditional love and support."

Brad has already sparked rumours he could be in line for an Oscar thanks to his performance in Heart of the Beast, with very little in the way of dialect and more resting on his ability to act without speaking.

And David is a massive fan of how Brad tackled the role, and prepared for the shoot.

He mused: " I think it has to do with his willingness to jump into material that isn't so traditionally engineered as a movie star role, or traditionally masculine.

"He's open to the deconstruction. And he has a lot of range, and I feel like he doesn't always have the opportunity to find those dichotomies in himself.

"Maybe there's also a kinship there in our understanding of traditional masculinity, the masks men are given in life to wear, and the fact that there's often a confused beating heart beneath them."