In order to accommodate a harness on Julie Andrews, most of Mary Poppins’ costumes had to be duplicated in a larger size for the flying scenes.

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According to the documentary, Supercalifragilisticexpialidocious: The Making of Mary Poppins, Disney personally flew to London in 1959 and met with Travers to once again try to convince her to let him adapt Mary Poppins. 

Speaking of the meeting, Travers reportedly said that talking to Disney was "like talking to a friendly, charming uncle who took from his pocket a gold pocketwatch and dangled it enticingly before your eyes."

In the documentary, Supercalifragilisticexpialidocious: The Making of Mary Poppins, composer Richard Sherman said the "key to the story" was the parents being so preoccupied with their own lives that there was a "need" for Mary Poppins to come, so it was essential that they start a little cold, especially the father.

Eventually, Travers signed off on the project and was billed as a consultant on the film.

At the time, Martin was best known for her work on stage in South Pacific, Peter Pan, and The Sound of Music. Meanwhile, Davis was fresh off her renowned work in Now, Voyager and All About Eve, and Lansbury had just starred in The Picture of Dorian Gray.

In an interview from 1973, Andrews recalled that Disney "started acting out the whole of the script of Mary Poppins" while visiting her backstage at Camelot. 

Then, Disney invited Andrews and her then-husband, Tony Walton, to Los Angeles to see the storyboards for the film. Walton, who was a costume and set designer, ultimately ended up working on Mary Poppins, too.

In her speech, Andrews said, "Finally, my thanks to a man who made a wonderful movie and who made all of this possible in the first place...Mr. Jack Warner."

Looking back on her career with the Hollywood Reporter in 2015, Andrews said the one thing she wished she had was a recorded version of her My Fair Lady to show her grandchildren. 

She also said that while she understood the decision, getting passed over for the film only reinforced the idea she had in her head that she wasn't "pretty enough" for movies.

Disney made Van Dyke screentest for the role, too, as a little joke. According to Andrews, "Word flew around the Studios that he had been hilarious, totally persuasive and completely unrecognizable."

He also said that to this day, British people will come up to him and tell him what a horrible accent he does as Bert in Mary Poppins.

He previously worked with Disney on Treasure Island and 20,000 Leagues Under the Sea. 

Bert and Mary walking arm-in-arm during "Jolly Holiday" was also one of the first pieces of choreography Andrews and Dick Van Dyke learned during rehearsals. Andrews said in her memoir, "I performed Mary Poppins’s demure, ladylike version of the step — but Dick flung his long legs up so high that I burst out laughing. To this day, he can still execute that step."

In one of the stories in the book, Mary Poppins spins a compass, and the Sherman brothers actually wrote songs for each of the places the compass lands on, like "North Pole Polka," "The Land of Sand," and more.

The melody for "The Land of Sand" was later used in The Jungle Book for "Trust In Me."

She continued, writing, "I literally did a lot of 'hanging around' between takes, and when I was suspended, the steel panels pressed on my hip bones, which became very bruised. Sheepskin was added, which helped, although it was barely enough, since I couldn’t look too bulky."

In her memoir, Home Work, Julie Andrews recalled, "For the tea party under the willows with the penguin waiters, a cardboard penguin was placed on the table in front of me. Once I’d established the sightline, the penguin was taken away, and when cameras rolled, I had to pretend it was still there."

"When I get over on the stage, I'd say, 'Where am I going to put my penguins?'," Thomas recalled in the documentary, Supercalifragilisticexpialidocious: The Making of Mary Poppins. He continued, saying, "Because I would get the film of Dick actually doing the dance, here's hit feet flying all around and stepping on my penguins. How are you going to know ahead of time where he's going to be and where Dick Van Dyke's going to be? So I was losing more penguins every day. I had them duck, and I had them jump, and I had them get out of the way anyway they could."

Since they didn't film the scene against a blue backdrop, it also didn't put a limit on the colors that could be used in costumes, like Bert's blue bowtie and socks. 

Van Dyke told Conan O'Brien in 2012 that he asked Gower Champion, who was the chroegpraher for Bye By Birdie, which he starred in on Broadway and in the film, to help him learn to dance. He's loved dancing ever since.

She added, "Our reactions were completely genuine ... It was very exciting."

In a documentary, Karen Dotrice recalled filming the moment, saying, "I don't know how many takes it took to walk up that smoke staircase because we're following Julie holding broomsticks, walking up this smoke screen staircase, but it was like sinking."

In the documentary, Supercalifragilisticexpialidocious: The Making of Mary Poppins, Tony Walton recalled, "Walt [Disney] himself came up with the technical approach to achieve the fact that they are laughing, [and] they are all floating about in the air."

In a documentary about Mary Poppins, Johns revealed, "I said to Walt [Disney], it might give me incentive, if I could have my own little number." Richard Sherman said that after that, Disney leaned over to her and said they just finished a number for Mrs. Banks, and she'll love it. At the time, Richard and Robert hadn't even written the musical number yet.

Richard Sherman revealed that Walt Disney felt the sequence wasn't necessary, and instead, after "I Love to Laugh," they all joke while drinking tea and then float down to the floor.  

Andrews said Walton's approach to the costumes helped her discover who Mary really was. She told Vanity Fair, "He said, 'Because I think that's what gives her pleasure. Very formal on the outside and a little bit wicked on the inside, so to speak.' And it completely gave me a clue as to her character. Big, big help for me."