Interesting that you bring up Charles Walters as I just finished the book, "Charles Walters, The Man who made Hollywood Dance." It was fascinating! I had no idea of all his directorial work, in addition to his choreographic contributions. He was quite revered by actors.
In regards to GIGI--According to the book, Walters offered to do a “family favor” for Freed and come in for four days to add subtextual characterization and choreography for “The Night They Invented Champagne.” Ultimately when the re-takes were shot, Minnelli was working on another film, so Freed placed the revisions on Walters and cinematographer Ray June. According to this books author, Walters primarily “smoothed all the rough edges in the picture.”
In addition, he re-tooled “Soliloquy” and tinkered with “The Parisians’ and “She Is Not Thinking of Me." It seems that Lerner’s memory was hazy and he erroneously insisted that Walters filmed the duet between Gingold and Chevalier (“I Remember It Well’). But Chevalier did not participate in the revisions, and Walters insists that he and the boulevardier had never met.
The audience reaction to the second preview ( after work by Walters) dramatically changed from appreciation to affection. When Minnelli received the Oscar for Best Director, he said, "it was about the proudest moment of my life."
Walters remained silent on the matter for almost two decades.