First, begin with this. The movie never tells this.
For me, as a teenager who got his first job in a record store in 1956, I was blown away by the My Fair Lady album, with Andrews’ portrayal of Eliza Doolittle, when Andrews was 21 years old. Much as I like the movie, I have never forgiven Jack Warner for not hiring Andrews to play her Broadway role. I realize that the movie needed a star, and Andrews was not the draw that Audrey Hepburn was, but it would have been better with Andrews. It deserved Andrews. I deserved Andrews!
Second, be prepared to see a movie about Walt Disney. If we are to believe this script, he was the most relentlessly cheery man in business history. Nevertheless, I enjoyed it. This was because of the other half of the story: P. J. Travers, who wrote Mary Poppins, and who is portrayed as one of the great dignified and eloquent curmudgeons of all time. She was probably worse than Emma Thompson portrays her.