"The first actor we saw for this role was Halle (Bailey). She shut her eyes and started to sing Part Of Your World, and I couldn't believe what I was hearing - she was so deeply connected to what she was singing about, it was so emotional and beautiful.
"And then I thought, "we've been doing this for five minutes, have we already found Ariel?" And we had - we saw hundreds of other actors after that, but Halle kept coming back in, and claimed the role as hers."
Marshall is one of Hollywood's go-to musical directors, having made the jump from the stage to the screen with Best Picture winner Chicago, before making several movie musicals for the House of Mouse including Into The Woods and Mary Poppins Returns. His trick to directing big-budget blockbusters on this scale, however, is to go back to his theatre roots, and apply those same tricks of the trade to a film set.
He explained: "Movie musicals are a hybrid of theater and film. You can't just walk in and start singing, you have to learn, and I always feel that my job as a director is to protect the actors, make them feel safe in a space where they can screw up and be terrible, and then get better without being judged.
"I want it to feel small and intimate, even though the scale of this project is massive in so many ways. But it's ultimately a small, beautiful story, and I never wanted any actor to feel the pressure of the filmmaking and the technical aspects of it - we didn't want those to take priority and start leading it over the story."
The film clocks in over the two-hour mark, much longer than Disney's 1989 animation, with Marshall and returning composer Alan Menken aiming to further flesh out the characters via brand new songs. This included adding a song for Eric, as well as more material for Ariel, who doesn't get the bulk of the original movie's music.
"It's surprising when you look back at the animated film; Ariel only had one song. A series of reprises, but one song.
"The great thing about working with Alan is that he's so open to looking at this. And he knew this was a different genre, a live-action film, so it needed a completely different approach.
"The most challenging thing in finding another piece for Ariel is that she's lost her voice - in film, you can create something where you're hearing those internal thoughts through music and song, which was they key to finding this piece for her. And it also accomplished a lot for us in terms of storytelling, because it's a montage of all her time on land, until she meets the prince for the first time."
Despite adding new twists to his world famous original compositions, Menken was incredibly open to reinventing his music for this new film, meeting up with the director, producer John DeLuca, screenwriter David Magee and the needs-no-introduction songwriter Lin-Manuel Miranda ahead of shooting to determine where new songs could help enhance the story.
Menken recollects: "We went through how the story was adapted by David, and the structure of it, finding out where there could be potential spots for a song - those decisions were made by character, by moment, and also by sequence of the score.
"So we chose the moment when Prince Eric has been told, "Don't go back out there, you can't," and he just thinks about this girl; what he sings is a love song to her, and it's a love song to the sea, and to the uncharted waters of his life ahead of him. This ended up being an important theme throughout the movie.
"And then there was the scuttlebutt, which was a surprise gift from Lin, because I gave him this Caribbean tune. And Lin did a rap over it that was so perfect, using the music, but it had this rhythmic pulse to it. It was just pure Lin-Manuel Miranda."
In other words, Menken is a collaborative spirit by trade, happy to give away his compositions for other musicians to put their own unique stamps on. But when it came to working with actors, Menken remained surprised at just how intimidated they all were to be singing for him.
"I'm not an intimidating person, but a composer can be intimidating to actors", he laughed. "I try to filter everything through the music scene as much as possible because they should have somebody they're really comfortable with in the room - I just want them to make it their own, and after that, if I have anything to say, I'll say it.
"But Halle, she's so talented, you can't take your eyes off her. The emotion is right there on her face and in her voice - she's an amazing Ariel.
"When we had our recording sessions, I was allowed to see the movie when it was just first put together very roughly. And Wyatt, the editor, kept having to bring over boxes of tissues for me, because when we got to Part of Your World, I was just weeping.
"Part of it is because I missed (lyricist) Howard Ashman, and then just remembering the innocence of what we put in there originally - and now look at what it has become!"
Ashman was diagnosed with AIDS in 1988, during production of The Little Mermaid, with songs composed up until his death in 1991 later used in other Renaissance era Disney animations including Beauty And The Beast and Aladdin. With this in mind, it's easy to get choked up at how his work will continue to endure for future generations thanks to this live action adaptation, and the talent of vocalists like Halle Bailey.
The Little Mermaid is released in UK cinemas on Friday, 26th May.Shop our Little Mermaid merch.
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