In order for a documentary to be great, is it obliged to be innovative? Some of the best documentaries I’ve seen over the past year – Stories We Tell, My Winnipeg, The Act of Killing – are innovative in one way or another, whether it’s through a self-reflexive examination of the process of documentary-making or an aesthetic approach …or simply through a daringly original idea.

Innovation is important, I would argue, but not necessary. Great cinema tests the boundaries of the medium, but great films can exist comfortably within those boundaries. Take Life Itself, Steve James’ biography of film critic Roger Ebert. It’s stylistically straightforward, composed of stock footage, talking heads and interviews, but that doesn’t prevent it from being an excellent, affecting documentary.

Partly that’s because of the strength of the subject matter – a consideration of Ebert’s legacy that transforms into a meditation on mortality – and the wealth of footage available. More significantly, however, Life Itself resonates because of its abiding honesty. Despite his obvious veneration of Ebert, James refuses to allow his film to become a hagiography; when we watch Chaz Ebert deny her husband his notepad – and thereby his voice – it’s a moment of painful honesty that transcends reverence.

20,000 Days on Earth, on the other hand, strives for innovation from its very concept, blurring the lines between fiction and documentary by following legendary Australian artist Nick Cave on his “20,000^th day on earth.” Where you might expect shaky handy-cam following Cave on his errands, we instead see a fictionalised representation of Cave’s life, as he visits friends, bandmates and an archive of his existence (plus encounters a few unexplained visions in the form of Kylie Minogue and Ray Winstone).

First-time directors Iain Forsyth and Jane Pollard’s ambition, sadly, doesn’t extend past the concept (which is muddied by cut-aways that chronicle the genesis of one of Cave’s songs, clearly over the course of many weeks). The photography – from The Double’s Erik Wilson – is undeniably impressive, but the content of the film remains mired in musical bio conventions. Cave is a fascinating figure, a true Australian icon, and an insightful and honest portrait of his life – including his complicated personal life, and his role as an author and screenwriter (which isn’t covered here) – would be something magnificent to behold. Instead, 20,000 Days on Earth’s insistence on innovation limits its effectiveness, while Life Itself demonstrates that great stories don’t require such embellishment.

Life Itself Rating:
20,000 Days on Earth Rating: