BJ Harrison, I don't think, was intended to be a consigliere as Tom Hagen had been. He is only a business lawyer. His lines are simply throwaway scene filler.

He is a wasted character. He says nothing, he does nothing, that sigificantly adds to the film. That's the great thing about GF I and II - there are no wasted characters.

The loss of Hagen IMO led, in a chain reaction, to most of the other problems of GF IV. The entire script had to be rewritten in a hurry, and the power-struggle/test of loyalty between Michael and Tom Hagen, which was, I am convinced, intended to be the main theme of the movie, completely written out and replaced by the hurriedly-slapped-together Vatican/Immobilaire story line.

I think if worse came to worse, FFC should have simply recast the role of Tom Hagen if he couldn't get Duvall to do it.

The loss of "Tom Hagen" the character completely disrupted the story line. To me it was more important to the movie that "Tom Hagen" be in it than it was for Robert Duvall to be in it. (Which is not for a minute to downplay Duvall's performances in GF I and II.)

I think Michael Moriarty (shown here from Law & Order) could have played Tom Hagen instead.