Jack shows a joyful, cheerful personality. In spite of his duty as a lamplighter (referred to as a "leerie") throughout the film, he shows a good personality to Mary Poppins and also joins alongside the three children, John, Annabel, and Georgie.
Behind the scenes
The "Hamilton" and "In the Heights" creator spoke to Slashfilm about the making of "Mary Poppins Returns" and expanded on Jack's role in it.
"Jack is a lamplighter. 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 Jack, a sunny chap who lights and extinguishes the streetlamps at the beginning and end of each day, and was once apprenticed to Bert. His role as the spiritual heir to Bert (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 film'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 it as he strolls along London. He then passes through Admiral Boom and Mr. Binnacle's house as he passes through that of the Banks family on Cherry Tree Lane. He then notices the kite that Michael used to fly as a little boy passing through him just as Georgie and Annabel chase after it. Just as Jack holds it, they notice Mary Poppins going downwards to warn the children to be careful just as Jack explains that he was their age when he first met her while working for a chimney sweep just as she leads them to return home. While she 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 they arrive towards Shamus' broken carriage. Having fixed it, he brings them across the park via carriage to the musical hall during the musical number "The Royal Doulton Music Hall". Jack then appears telling the animal audience that Mary will perform on stage. The next day, he encounters Jane who is also greeted by Mary just as Jane introduces herself to him. Mary, Jack, and the children leave the house as Admiral Boom and Mr. Binnacle fire their cannon to mark the signal just as the group arrives at Topotrepolovsky's All Repairs Large and Small Fix-it Shop to fix the broken bowl where they encounter Topsy who refuses to let them in. Barging into her shop, they encounter her who refuses to fix the bowl at first because of Second Wednesday. She then introduces them in her shop about everything going topsy-turvy in it, just as she agrees to accept Mary's request to fix the bowl just as they leave.
Walking across London, the group encounters Jane again where Jack encourages her to ride on his bike just as Mary and the children arrive at the bank to see William Weatherall Wilkins who would help with the Banks family's house. As Mary plans to take the children home on a foggy night, they find Jack again where he performs the musical number "Trip a Little Light Fantastic" just he 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 on 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 them and Mary get to the bank and stop Wilkins to turn back time. Jack and his crew of lamplighters work together to help Mary and the children do so. Reaching Big Ben, Jack gets help from the lamplighters to reach the top. As time is running out, Jack tells Angus to mark the signal so he can reach the top, and he is launched toward it and then enters it. Turning off the lights inside, Jack is unable to reach the hands as it is less than one minute before midnight. Mary manages to 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 that of Bert, a character played by Dick Van Dyke in the first film.
- Van Dyke, however, later returned as Mr. Dawes Jr., making him one of the few actors from the original film to participate in the sequel after more than fifty years of waiting for a continuation to the original film.