The "Hamilton" and "In the Heights" creator spoke to Slashfilm about the making of "Mary Poppins Returns" and expanded on Jack's role in the film.
"Jack is a lamp-lighter. He apprenticed under Bert from the original films so he knows all about Mary," said Miranda. "So it's really nice to play the Bert position in this film. You kind of get to go on all the fun adventures with the Banks family."
Mary Poppins is aided at every step by her old friend Jack, a sunny chap who lights and extinguishes the streetlamps at the beginning and end of each day, and was once apprenticed to chimney sweep Bert. Jack's role as the spiritual heir to that Dick Van Dyke's character (with a slightly more convincing Cockney accent) is clear from his opening number, (Underneath the) Lovely London Sky, which sets the scene with a vivid sense of place and a dash of everyday magic. Miranda was the casting wild card here, but the Hamilton creator is a snug fit, bringing a pleasingly gentle manner and a twinkle in his eye that make him just as beguiled by Mary Poppins' mad skills as the children.
Miranda leads the movie's splashy centerpiece number, a direct counterpart to Bert's rooftop Step in Time called Trip a Little Light Fantastic, performed with a large chorus of Jack's fellow lamplighters on bicycles, wielding the ladders they use on the job. Rob Marshall, who also choreographed the film with John DeLuca, nods back not just to vintage Disney here, but also to the MGM movie musical in the number's exuberant build of layer upon intricate layer of dance moves.
Jack first appears at the beginning of the film, singing the musical number "(Underneath the) Lovely London Sky" where he is introduced singing the song as he strolls along London. He then passes through Admiral Boom and Mr. Binnacle's house as he passes through the Banks family's house at Cherry Tree Lane. Jack the notices the kite that Michael used to fly as a little boy passing through him just as Georgie and Annabel chase after the kite Jack was following. Just as Jack holds the kite Georgie is hanging onto, the two notice Mary Poppins going downwards who warns the children to be careful just as Jack explains that he was the children's age when he first met working for a chimney sweep just as Mary leads the children to return home. While Mary tells Georgie to fix the broken kite, she talks to Jack outside where he tells her that he has seen the Banks family ever since.
Later, Jack, Mary, and the children arrive in the animated sequence where all the inhabitants are anthropomorphic animals just as the group arrives towards Shamus' broken carriage. Having fixed the carriage, Shamus brings the group across the park via carriage to the musical hall during he musical number "The Royal Doulton Music Hall". Jack then appears telling the animal audience that Mary Poppins will perform on stage. The next day, Jack encounters Jane who is also greeted by Mary just as Jane introduces herself to Jack. Mary, Jack, and the children leave the house as Admiral Boom and Mr. Binnacle fire their cannon to mark the signal just as Jack, Mary, and the children arrive at Topotrepolovsky's All Repairs Large and Small Fix-it Shop to fix the broken bowl where the group encounters Topsy who refuses to let the group in. Barging into Topsy's shop, Jack and the group encounter Topsy who refuses to fix the broken bowl at first because of Second Wednesday. Topsy then introduces the group in her shop about everything going topsy-turvy in her shop, just as she agrees to accept Mary's request to fix the Royal Doulton bowl just as the group leaves the shop
Walking across London, the group encounters Jane again where Jack encourages her to ride on the bike just as Mary and the children arrive at the bank to see Mr. Wilkins who would help with the Banks family's house. As Mary plans to take children back to Cherry Tree Lane on a foggy night, the children find Jack again where he performs the musical number "Trip a Little Light Fantastic" just Jack shows Mary and the children across the street encountering a group of lamplighters (reminiscent of the chimney sweeps seen in the first film's musical number "Step in Time") while Mary and the children encounter the lamplighters participating in a dance number. Jack and the lamplighters dance together in Cherry Tree Lane just as Mary and the children return home.
That night, Jack later prepares to put in the objects as the Banks family is moving to somewhere else while Mr. Binnacle watches over this. Later when the children notice that they are moving, Jack manages to help Mary and the children get to the bank and stop Mr. Wilkins to turn back time. Jack and his crew of lamplighters work together to help Mary and the children stop Wilkins. Reaching the Big Ben, Jack gets help from the lamplighters to reach the Big Ben. As time is running out, Jack tells Angus to mark the signal so he can reach the top, so he is launched toward the top of the Big Ben's clock and then enters it. Turning off the lights inside, Jack is unable to reach the clock's hands as it is less than one minute before midnight. Mary manages stop the clock back to five minutes before midnight, stopping Wilkins' plan.
With Wilkins taken away, the children retain their home back just as Jack attends the spring fair alongside the Banks family. He last appears during the end of the film where he sings a reprise of the musical number "(Underneath the) Lovely London Sky" just as Mary leaves via umbrella.
- The tone that Lin-Manuel Miranda uses when playing Jack has a great imitation of the tone of Bert, a character played by the legendary Dick Van Dyke from the original film.
- Van Dyke, however, later returned as Mr. Dawes Jr., making him one of the few actors from the original film participate in the sequel after more than fifty years of waiting for a continuation to the original film.