Alec Guinness wrote the screenplay. Already an actor of great repute, in doing so he was in a very dominating position. The film gives the impression of a much less collaborative affair - writer/director/star - than is usual, or successful. Very hard for director to question a portrayal when the star can truthfully say that he knows the character far better than does the director.
Guinness chose a deep croaky voice. He consequently lost all musicality - most obviously when he sings but throughout his voice is an inexpressive monotone. The dialogue lacks sparkle further dulled by his monotone. There is a problem in any case of portraying a talented but inarticulate artist - how do you indicate talent or even genius? Apparently based on the writer Dylan Thomas, whose drunkenness was companionable (Richard Burton once a companion?), there was no doubt wit in their conversation. Here the painter is mainly rascally, the paintings shown don't particularly impress either intrinsically or by the way they are treated. All that is left is the implication that for someone so badly behaved yet to still be sought after, must have a great deal of talent. The film fails to show people being won over by his pictures and forgive his trespasses - that's a failure of direction.
Given the great talents involved, it is less than it could have been. But given these talents, it should not in anyway be patronised. Even great artists get it wrong sometimes, it doesn't affect their greatness or my admiration.