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Suckers. He was faking it all along. The monosyllabic, bearded, totally spaced-out Joaquin Phoenix has been exposed as a fraud by his brother-in-law and co-conspirator Casey Affleck.

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The change of persona from responsible Phoenix to far-out Phoenix was first presented to the public . Although , the performance succeeded in fooling most - including, it seems, Letterman.

This was followed by the documentary being shown at this year's Venice and Toronto film festivals before going on general release. It is directed by Affleck and records the absurd life and ambition of Phoenix as he attempts a career change from accomplished actor to wannabe hip hop star.

Affleck had insisted at all the press junkets that the film was for real, a true account of a man in crisis/hippy heaven. But , and that the film is in fact a mockumentary in the spirit of previous spoofs which sit on a spectrum between Spinal Tap and Sacha Baron Cohen's Borat.

(On which subjects - that is, music films, character observation and Baron Cohen - the man who became Ali G, Borat and Bruno is now to become Freddie Mercury in a new biopic of the Queen frontman currently being written by Peter Morgan, he of Last King of Scotland and Frost/Nixon.)

In all of these cases - fake Phoenix, for-real Ali G, flamboyant Freddie biopic - the central conceit is for the performer to convince others that the human being they are watching really does or has existed. They are not presented as a fictional person within a story, but as genuine inhabitants of our world.

Their currency is truth. Borat and Ali G worked because Baron Cohen was able to convince the real people in the real world with whom he was interacting that he too was real. And a bio-pic relies on the audience believing that what it is being told is true.

They are, broadly speaking, documentaries: chronicles of life. And yet they are all made up, false, pretence. But then does any documentary, in whatever form, ever tell the whole truth? The hour or 90 minutes you see is of course an edited version of events, constructed in such a way as to tell the story the director wishes to tell. It is necessarily subjective.

Joaquin Phoenix and Sacha Baron Cohen have, in their way, both been probing and questioning the truth of documentary - as I will be doing in greater detail next week.