By comparing the novel, The Killer Angels, to the movie, Gettysburg, how did Hollywood take a fictional novel and change it?

There is little documentation to the thoughts of the main
characters  during the days leading up to the Battle of Gettysburg, so author Michael
Shaara fictionalized this aspect of his Pulitzer Prize-winnning novel. As for the film
version of Gettysburg, the screenplay actually maintains most of
the characters, dialogue and scenes found in The Killer Angels.
Much of the dialogue, particularly several of Chamberlain's speeches, are word-for-word
from the book. Though the end result of the movie was a far cry from the outstanding
novel from which it originated, the film did maintain the general tone and feel
of Shaara's book. My biggest complaint about the film was the casting of Martin Sheen as
Robert E. Lee and the terrible, fake beards worn by many of the characters, particularly
that of Tom Berenger, who played James Longstreet.