The fine art of the mockumentary

Casey Affleck's spoof film 'I'm Still Here' follows a grand tradition of blurring the line between fact and fiction

The jury's still out on Joaquin Phoenix. Did the Walk the Line star quit acting to try his hand at becoming a hip-hop musician as a ruse to help his brother-in-law Casey Affleck make a much-hyped documentary about celebrity, friendship, media intrusion and the cost of fame?

Affleck avoided answering the question both in his film and when talking to the press in Venice, where his movie premiered on Monday. And why not? He should be praised for keeping the audience guessing. The rumours of a hoax are a major component of the film; speculating on what is and what isn't real is all part of the fun.

I'm Still Here joins Banksy's Exit Through the Gift Shop and Catfish, co-directed by Ariel Schulman and Henry Joost, as not only one of the best documentaries, but one of the best films of the year. And in all three, the audience is never told explicitly if what they are watching is truth or fiction.

In Exit Through the Gift Shop, Banksy plays with the question of his identity in a film about the attempt to discover who the artist really is. Much of the hype surrounding the film before it premiered in Sundance centred on whether he would reveal himself. In the event, the artist appears on camera shrouded in darkness and speaks in a scrambled voice. It doesn't matter that there is no big reveal, what carries the film is its oral history of street art and searing social critique.

Catfish, one of the break-out hits of this year's Sundance, has the advantage of not featuring anyone famous. Already labelled "the perfect documentary", it's the story of a friendship that develops on Facebook when an eight-year-old girl from Michigan emails the New York photographer Nev Schulman, brother of the film's director, with an immaculate painting she has done from one of his photographs. The directors shoot the film as if it's all happening in real time but it's hard to believe some of it isn't staged. Quite rightly, though, the directors refuse to talk about this – the uncertainty is part of the film.

The fun of mockumentaries lies in their attempt to convince us that what we are watching is the truth. This is Spinal Tap, the granddaddy of the genre, has taken the joke so far that the band now appears in concerts as if they always were real rock legends. Sacha Baron Cohen mixed fact with fiction with his three creations for The 11 O'Clock Show; Ali G, Borat and Bruno. The laughs come from him suckering unsuspecting individuals into believing he is for real. When he messed with that formula for a movie, Ali G Indahouse, it was an unmitigated disaster. Borat, on the other hand, remains his crowning glory because it's the work in which most of those featured believe he is a real man from Kazakhstan.

I'm Still Here tries an even bigger trick. The desire is clearly to split audiences into two categories, those who think it's a joke and those who don't. It's a magnificent ruse. On the one hand, it's hard to believe Phoenix is really such a jackass and that his brother-in-law would expose him as such. What's more, anyone who has seen Walk the Line knows that Phoenix has talent – both as an actor and a musician – in spades, which makes his disastrous attempts at hip-hop unlikely. On the other hand, his actions have affected others in a very real way. His last film, Two Lovers, never had a chance in America after Phoenix made a fool of himself on Letterman with his rapper stylings. The film died a box-office death – not much of a joke for its director, James Gray. Can he ever forgive Phoenix?

Whatever the truth, let's hope Affleck and Phoenix do not veer from their position, asserting that everything on screen is real. It makes a better story. Phoenix has spent two years living as this persona, the character on screen takes his name. Truth has merged with fiction.

'I'm Still Here' is out on 17 September