When you apply the logic of musicals, the story of an ensemble cast member volunteering to play the starring role with just four hours of rehearsal seems perfectly at home. After all, High School Musical always made it seem easy.

But, when you take off the sparkly showbiz sunglasses and remember this is, in fact, real life we're talking about, then what occurred at Paddington the Musical this week becomes far more remarkable.

Rose Mary O’Reilly was buying coffee on Monday morning when she got the call to say that that evening's performance of Paddington was likely to be cancelled.

Victoria Hamilton-Barritt, who plays the villainous taxidermist of the show Millicent Clyde, was on holiday abroad and all three of her covers were unable to perform. That is, unless, someone else could step into those leather heeled boots...

"I was never going to say no," she said, in an exclusive chat with Radio Times, "I was scared to say yes, but I knew it wasn't going be a no."

O'Reilly's role as a Swing – an off-stage cast member who covers multiple ensemble roles – meant she was used to adapting quickly, but she had never covered a role of this magnitude.

Millicent Clyde has quickly become one of the most recognisable roles in Paddington, owing to Hamilton-Barritt's Olivier Award-winning voice and the standout song Pretty Little Dead Things – so for O'Reilly, only in her second theatre job post-drama school, the challenge was the size of one of Clyde's stuffed elephants.

"No one ever pressured me to say yes. There was never a sense of 'you do this or you cancel the show'. But whenever I've been asked 'if you could play anyone in the show, who would you play?' I would always say Millicent, so to hear that on the phone... I already knew my answer."

O'Reilly's parents, visiting from Cork and scheduled to head back that night (again, musical logic), also knew this was a no-brainer for their daughter, so promptly cancelled their flights to support her for the day ahead.

Rehearsals started at around 3pm for a 7pm show, which meant it was all guns blazing. O'Reilly describes running through songs, lines and tech as a "complete blur" with dozens of cast and creative members getting involved to help.

"I just kept hugging people. I was terrified and this company means so much to me, they genuinely are like my family and my friends, so when I saw all of their faces I just wanted to hug them, and not once did anyone make me feel like I couldn't do it."

The amount of absences meant that night's performance would officially be a 'cut show', with a reduced ensemble cast on stage. This meant even more work, as choreography and formations had to be adapted. O'Reilly said her comfort was made the first priority, and as such she was offered to appear on stage with a script, which she refused.

"I just thought 'If I'm going to do it, I'm doing it properly, I want to at least try my very best to play Millicent, especially for Pretty Little Dead Things.' Everyone loves it, so I wanted to the character and that song just the tiniest bit of justice."

The actress was then put in a combination of costumes from the various other Millicents, and, with all the manic changes, wrapped up rehearsal just 10 minutes before the scheduled showtime.

"I was just in disbelief. I just kept saying 'Oh my god, oh my god'," she said, "But my dressing room was full of support from people that were helping me stay calm, then when I got down to the wings, I just kind of went really calm and really still and I was just determined."

However, even after the show began, she said it "took a village" to keep everything running smoothly.

"Erica Stubbs (Associate Choreographer) and Toby Murray (Associate Director) never left my side, I had no clue where to go to get changed so the dressers would just grab me, and at one point stage manager Sean guided me off stage, Tom Eddin came up to my dressing room in the interval to run lines, and they were all just fantastic, and I had so much fun."

For O'Reilly, starring on the West End has been a dream since she was four-years-old, and so she gladly welcomed all the chaos the day had brought.

Her gutsy move is the perfect example of asking "why not me?" hence why it received so much applause on the night and in the days since.

Following the evening, producers Sonia Friedman and Eliza Lumley released a statement that read: “We are beyond grateful to Rose who, when it became clear that we might have to cancel last night’s performance, extraordinarily volunteered to step into Millicent Clyde’s shoes.

"She went on completely off-book, knowing every word, every musical note, every move, and the complete character arc. What she achieved was quite simply astonishing. It was also an extraordinary company effort – fellow cast members, our music department, associate creatives, stage management, wardrobe, wigs, crew, sound and the SFP team all pulled together to support her and make sure the performance could go ahead.

"Rose quite literally saved the show and, in doing so, gave 1,100 people a performance they will never forget. It was one of those very rare nights in the theatre when everyone in the building knew they were witnessing something truly special.”

Looking back on it now, O'Reilly said the experience has given her much more appreciation for her role as a Swing: "I think the fact that I am a Swing massively helped me do what I did.

"Everyone says Swings and Covers are the absolute backbone of this industry for what they can pull out on short notice, so I'm so honoured to be one and to be surrounded by such incredible swings to work with.

"At the end of the day, the night was a dream come true, but I still want to do an incredible job with what I'm doing now. I care so much the show and I just always want to work my hardest at it.

"I would just love to tell my four-year-old self that if you really believe in yourself and you take it seriously, work hard, and you find good people. It's possible."

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