'Mary Poppins Returns': Behind the scenes of how new film was made

Stars Emily Blunt, who plays Mary Poppins, and Lin-Manuel Miranda, who plays Jack the lamplighter, describe how they approached their roles in this next chapter that features brand new music.
7:00 | 12/07/18

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Transcript for 'Mary Poppins Returns': Behind the scenes of how new film was made
Reporter: It's a movie sequel more than a half century in the making. Mary -- Poppins! Close your mouth, please, Michael, we are still not a codfish. Someone said to me, I didn't realize how much I needed her to come back until you came out of the clouds. It's just so hopeful and so joyful. Reporter: 54 years after the debut of the Disney classic with Julie Andrews -- Mary Poppins, practically perfect in every way. Reporter: Mary Poppins returns comes to theaters this December 19th. An all-new story of the nanny and her fantastical adventures starring Emily Blunt, lin-manuel Miranda, and more of the Mary Poppins magic that's long uplifted audiences. You have one of the great entrances in recent cinema and you're part of it as well. As I live and breathe. I had the best part because I got to stay on the ground. I got to just reel her in. Just watch me pretend not to be terrified. 50 feet in the air, dangling from a crane. Did you ever have one of those moments where you're saying, how did I get here? Oh, yeah. That was a huge moment where I was like, oh my god, I'm Mary Poppins. Reporter: Best known for the best picture Oscar win jeer Chicago," director rob Marshall cast Emily as the British nanny who's practically perfect in every way. How'd she do that? Do what? What happens when you're sitting at your home and you get a call saying, I want you to play Mary Poppins? Which happens daily. Rob Marshall was beautifully ceremonious about it when he called me. It felt like he was sort of proposing to me. It was a little bit of a drumroll. I said, I'd love you to play the role of Mary Poppins. He said, you know, Disney's most prized possession is Mary Poppins. There was sort of silence for a moment. I heard a big sort of intake. I felt my hair blow back with a combination of panic and joy. And thrill, really. We started talking about the character and she goes, oh, and by the way, it's a yes. Jean banks, still inclined to giggle, I see. Reporter: For the sequel she made it a point to make Mary her own. Did you look at the first film when you got cast? I did the opposite. Instead I dove into the book. And she's quite different in the books. She's completely eccentric and so rude. My goodness, Annabel what have you done to your clothes? You could grow a garden in that much soil. Deliciously rude and vain. Don't you remember that? Reporter: The story is set 25 years after the first film. Jane and Michael banks are all grown up. Michael has three kids of his own and is deep in crisis. Everything's falling to pieces. Reporter: Everyone on cherry tree lane needs Mary pop Kins' help again. The bank is now demanding that you payback the entire loan in full. The entire loan? So to have sort of that central problem for Mary Poppins to come and help, after 54 years, it needed to be a big deal. The one, the only -- Mary Poppins! Thank you. Thank you very much. Very, thank you. Reporter: As Dick Van Dyke's bird did in the first film. All right, I'll do it myself. Reporter: Lin-manuel Miranda's jack is a big part of Mary's fantastical adventures. ??? You would just look up ??? I like to picture, he apprenticed under Burt, he knows how magical Mary Poppins is. Most kids forget, jack's never forgotten. Good to see you, Mary Poppins. How big a dream would it have to have been to imagine being in a Mary Poppins movie? Audacious. It's the only word for it. You never think there will be another Mary Poppins film. Good heavens, it really is you. You seem hardly to have aged at all. Really. How incredibly rude. One never discusses a woman's age, Michael. Who is she there to save? Michael, really. What brings you here after all this time? Same thing that brought me the first time. I've come to look after the banks children. Us? Oh, yes, you too. I think she comes to look after the banks children, but ultimately I think it's about him. He's a banks child as well, you see. Reporter: It's the children that get to go on those magical adventures. ??? Some people like to dive right in can you imagine that ??? She very much wants people to rediscover the joy and the hope that children have. There's a moment right before she goes into this big underwater adventure that she says, off we go! She's more excited than the kids. One of my favorite lines she says is, everything is possible. Even the impossible. Reporter: Impossibly enough, Mary Poppins returns features a cameo from Dick Van Dyke, now 92 years young. There was this really beautiful moment where I was talking to him. He took my hand. He goes -- ??? it's a jolly holiday with Mary ??? and I was just like, you're going to make me cry! You can't do that to me! Reporter: The original songs for Mary Poppins returns written by mark Sheaman and Scott Witman. Mary Poppins returns is our love letter to the original film, the original filmmakers. ?????? It's all in there. So I think the audiences can rest assured that when they go see Mary Poppins returns that they're not going to get cheated out of anything they may want to hear or want to see. Then they're going to get a whole lot of new stuff. Reporter: From wistful nostalgia to childlike wonder, Mary Poppins returns is looking to set off waves of emotions. For those who still treasure the first film, and a spoonful of sugar. ??? Let a spoonful of sugar help the medicine go down ??? Reporter: And those brand-new to her magical world. I knew I couldn't possibly. D flat major. When I was a kid I remember the animated sequences. I watched it as an adult with children of my own. I realized, this is about family. Kids who see this movie are going to be wowed and amazed by the spectacle of it all. I think adults are going to go, man, our childhoods went like that. And hopefully for two and a half hours you're back in your childhood again. Reporter: For "Nightline," I'm Chris Connelly in los Angeles. A big thank you to Chris and Mary Poppins returns, opening in theaters on December 19th.

This transcript has been automatically generated and may not be 100% accurate.

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