What "I'm Not There" represents are the various personae created by Bob Dylan at various stages of his life, and through his music more than through the known facts of his life. So, we see a young black boy who represents Dylan's interest in Depression era folk music, called, not insignificantly, Woody Guthrie. Then there is the earnest young poet, enraptured with Rimbaud. And the movie star, and, memorably, the electric guitar-playing Dylan, portrayed by Cate Blanchett. Richard Gere doesn't have a lot to do beyond look wise and folksy, playing the late era Dylan.
You can't help be reminded of how wonderful the music is.
The movie is a commentary on fame, and identity, and authenticity. It is rich and complicated and a little too long but definitely worth your time.
I found the scenes with Heath Ledger almost unbearably poignant.
Also, although everyone is talking about Cate Blanchett, who has much the showiest role, we should pay attention to Christian Bale, whose portrayal of both the "folky" Dylan and the "Christian convert" Dylan, is quite eerily accurate.