When it came to his ideas about movie-making, Walt Disney wasn’t used to hearing the word “no”—much less “No, no, no, no, no!” But loud protests were exactly what he got from Pamela L. Travers, author of the Mary Poppins books. First, she resisted for over 20 years before giving him the movie rights. After that, she balked at almost everything he wanted to do. Use original songs? Cast Dick Van Dyke? Include cartoons? No, no, no!

Saving Mr. Banks tells the story of Disney and P.L. Travers’ tense relationship, as each sought creative control in making Mary Poppins. Their artistic visions differed drastically. “The original Mary Poppins was not cheery at all,” wrote Valerie Lawson, Travers’ biographer. “She was tart and sharp, rude, plain and vain.” In contrast, Walt cast the beautiful Julie Andrews and had her sing, another point of contention.

Because Saving Mr. Banks is a Disney movie, it will probably reflect well on him and end happily. But in reality, there was no neat resolution: Travers felt bitter about Disney for the rest of her life and refused to sell him any more stories.

Tom Hanks will star as Walt Disney and Emma Thompson as Pamela Travers, whom she called “the most difficult person I’ve ever played.” Saving Mr. Banks opens to limited release on December 13 and wide release on December 20. Watch the trailer below: