The Mary Poppins author was spikey, defensive, quite rigid and turned her back on Oz and even changed her name. As the film develops we find out she is actually emotionally damaged from her childhood and as the Jesuits say give me the 7 year old girl and I will show you the woman. Her father looked to be promoted beyond his abilities and interests and turned to drink. literally drinking himself to an early death when the author was a girl. She still loved her father dearly and it came through in her books.
Walt Disney was clever enough to see the author was emotionally damaged, but instead of trying to bully her (which he was renowed for doing to his staff) or rejecting her, his sort of took her underneath his wing, so he was able to gain the rights to film her books.