The 57-year-old filmmaker - whose credits include 'Into the Woods' and 'Chicago' - was apprehensive about remaking the 1964 classic, which starred screen legend Dame Julie Andrews as the magical nanny, because he knows how much the original was adored and he felt the 'Devil Wears Prada' star was the only person able to do the part justice.
He told the latest issue of Empire magazine: ''None of us would have touched a remake in a million years.
''The original film means a lot to all of us.
''I don't know if we would have done this movie with Emily.
''I think it would have been almost impossible.''
Rob explained that the only way to approach the new film was by developing a story around Michael Banks (Ben Whishaw) struggling to bring up his three children following the death of his wife and needing Mary to return to work her magic.
Speaking about the plot, which is based on the book series by P. L. Travers, he said: ''For the kids (Michael and Jane, played by Emily Mortimer), things have also changed.
''They've almost become the adults now and their childhood is escaping them. So I thought, 'Okay, there is a reason.'
''Mary Poppins comes back to help fill that hole in the family.'''
Emily credits Rob for helping ease her fears about living up to the iconic performance of Julie.
She said previously: ''It was the most magical experience, because Rob Marshall is so loving and nurturing. He made it feel like an intimate experience and less of what I worried it would be, which would be to try and reimagine this character that is so beloved, and played by an iconic actress like Julie Andrews.''
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