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 (Lin-Manuel Miranda), 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 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. Marshall, who also choreographed the film with 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.