Larry Parks plays Al Jolson but he does not try to sing like him. He is always dubbed by Jolson himself, which looks a little strange at times but he lip syncs well enough, so it is not all that obvious. When it is the extreme close ups though Parks face just does not fit Jolson's voice. A lot of the performance is spent singing so for portions of the film this is all Parks is doing.
Parks is technically limited by the fact that he is in an old biopic therefore he can never really get under the skin of the character that well. Parks though never goes off on his own to put more into the performance that what is in the script either. He does nothing subtle at all in this performance, but that is not exactly a problem. Jolson is shown just on the surface and that is how Parks plays him.
He has the right sure of himself gestures. He smiles, and says a lot of things very excitingly. He has right amount of showmanship, and he is romantic enough with Evelyn Keyes. There is scene that requires a little more is when he talks about being a workaholic. He does an okay job in this one scene but he still does not show a deeper side of Jolson which he could have in this one scene.Parks basically does what he needs to do but that just is not that much of a feat.