These facts came via the documentary Supercalifragilisticexpialidocious: The Making of Mary Poppins, unless otherwise stated.
1. Walt Disney first began pursuing the rights to the book in 1938, but was rejected by the book's author, P.L. Travers.
2. Over the years, Walt Disney continued to make offers for the book's rights. In 1959, he and Travers finally met in person in London. She agreed to option the book to him.
3. Iconic Walt Disney Studios' songwriters — and brothers – Robert Sherman and Richard Sherman, worked on the music for the film for 2.5 years, unaware that the studio did not have the full rights to the book.
4. Walt Disney originally considered Angela Lansbury, Mary Martin, and Bette Davis for the role of Mary Poppins.
5. Robert Sherman was the first to consider Julie Andrews for the title role after he saw her and Richard Burton perform a song from Camelot on The Ed Sullivan Show.
At the time Andrews and Burton were both appearing in the show on Broadway.
6. Julie Andrews agreed to make the film after she was rejected for the role of Eliza Doolittle in the film version of My Fair Lady.
Andrews' had originated the role of Doolittle — to much critical acclaim — playing her on both Broadway and London's West End.
Julie Andrews was considered a shoo-in for the film, but then-Warner Bros. President Jack Warner decided she was too much of an unknown to carry the film and cast Audrey Hepburn instead.
7. The film marked Julie Andrews' big-screen debut.
8. David Tomlinson, who played Mr. Banks in the film, also provided the voice of Mary Poppins’ parrot head umbrella.
9. At the beginning of the film, some of the nannies waiting in line to apply for the nanny position were actually stuntmen in drag.
10. Dick Van Dyke’s notorious Cockney accent was partially to blame on his Irish vocal coach Pat O'Malley, who, according to him, “didn't do an accent any better than I did.”
11. This was the second of three Disney films that Matthew Garber (Michael Banks) and Karen Dotrice (Jane Banks) co-starred in.
12. The "Step in Time" dance sequence was originally slated to be nine minutes long, but the director tried to cut it down to two after fearing it was too long. When Walt Disney found out, he actually decided to add more elements to the sequence, extending it to 14 minutes.
The cast rehearsed the "Step in Time" dance sequence for six weeks.