Bob Dylan.
no shit... but why is Cate Blanchet playing a dude... and how can like 6 people be playing the same dude???
We might just as well ask how only one actor can be an authentic representation of a person, of one person's multi-colored life. For example, I think of myself as a very different person from the person I was in junior high, high school, college, post-college, pre-children, post-children. Biography is problematic - it tries to represent a person, a person's life, by way of narrative, by way of key events, but who decides what that narrative is, what the key events are, what things and moments define a person? It makes a great deal of sense to me to offer a representation of a person (a representation that is very consciously a representation, not the last word in truth/reality) by way of bits and pieces, by way of different actors' interpretations, by way of different actors, by way of different narrative lines. Our lives can be told from many different perspectives - that's why, I think, there's usually not just one biography of a well-known person, there are several, each often interesting or valuable in their own ways. I think the film is conscious of that - the title admits that even in all the different perspectives and interpretations, Dylan may not even be in the film at all.
It's particularly interesting to think about our perceptions of a person like Dylan, so public in so many ways that I think his fans, the public want to believe they know him, that they have a correct interpretation of him, or that they have a handle on who he is - but what a conceit that is - to assume that because parts of a person's life, parts of his words and music are visible, that we can "know" him. I love the scene in which the fans are enraged by the unexpected electric guitar - they expected a certain person, the folk singer they'd assumed him to be (whatever "folk-singer" meant to them), but he slid out from under that label, in defiance of that interpretation, that particular slot. The film, I think, does a kind of homage to Dylan's right to be who he is without the perceptions of others being forced upon him, as those perceptions have been forced on him in his career and as he has been considered public property in some ways.