Creator Backlash: P.L. Travers called the second chapter of the first book, "The Day Out", the worst Mary Poppins story she ever wrote. However, she apparently never said how much of her hatred, if any, stemmed from Walt Disney using its concept of jumping into chalk drawings as a way to shoehorn animation into the movie.
The Disney movie
Creator Backlash / Disowned Adaptation: P.L. Travers absolutely hated the film, due to it's changing of several elements of her original stories (In the books, Mary is a complete Jerk Ass throughout) and the inclusion of the animated sequence. So much so, that at the premiere, she approached Walt Disney before it's first screening, demanding the sequence be removed. Disney refused, and later, when the film had concluded, Travers broke down crying, not because the people hated the film, but because they were giving it a standing ovation.
Development Hell: Walt Disney had first considered making a live-action adaptation of Mary Poppins as early as 1938, but P.L. Travers didn't think it could do justice for her books. Mainly because Disney hadn't done any live-action films at the time.
Fake Brit: Dick Van Dyke. That accent. Ironically, he also plays Mr. Dawes Sr, doing so brilliant a job that many viewers don't even realize it's him.
A possibly apocryphal tale is that the two child actors didn't know it was Dyke until his name unscrambled in the credits at the premiere!
In addition, Constable Jones, the police officer who brings Jane and Michael home, and is summoned to investigate the disappearance of Mr. Banks is none other than both Jeeves and Merv Griffin's sidekick/announcer, Arthur Treacher.
Last of His Kind: This was the final onscreen role for both Treacher and Jane Darwell, the Bird Lady.
Uncle Albert is played by Ed Wynn, who appeared in numerous Disney films, most notably being the voice of the Mad Hatter in Alice in Wonderland.
In addition, the film's vocal coach, J. Pat O'Malley, also appeared in numerous Disney films; he was also in Alice in Wonderland, voicing Tweedledee and Tweedledum, and he was also Colonel Hathi in The Jungle Book.
Hermione Baddeley, who played Ellen, the Banks' parlormaid, also was the voice of Madame Adelaide Bonfamille in The Aristocats.
In addition, Thurl Ravenscroft, who voiced the hog in the barnyard scene in the animation sequence, also appeared in The Aristocats as the Russian cat.
Director Robert Stevenson spent much of his career directing live-action Disney films.
In addition, music editor Evelyn Kennedy also worked on various Disney films, live-action and animated alike.
Walt was negotiating to have the rights to film Bedknobs And Broomsticks in case negotiations for the rights to Mary Poppins fell through.
The book had a chapter in which Mary Poppins and the children use a magic compass to visit different countries. The Sherman Brothers wrote a wide assortment of songs to use for their journey, but the chapter ultimately went unadapted.
Although Julie Andrews was marvellous in her Oscar-winning turn as the magical nanny, you can't help but wonder what might have been if an earlier choice for the role had been the last; Angela Lansbury was seriously considered before Andrews was finally cast.