The 14-minute sequel will catch up with the characters 14 years after the classic Christmas film was released.
Speaking to BBC Somerset Freud said: “Richard was desperate to find something we could make that we could show in Red Nose Day UK and in Red Nose Day USA which is something we do now in May. He said to me one day, ‘What if we do a mini sequel?’ and I went, ‘Literally the worst idea you’ve ever had.’
“It’s such a lovely film, you don’t want to mess with it and it would be tacky and horrible, don’t even think about it! But he ignored me.”
She continued: “On this instance he was completely right and I was completely wrong, but it took a long time to get the script there,” continued Emma.
“It was a very, very bad script when he first started doing it and we had to work harder on it than usual because it was a kind of weird idea, getting those characters, making it 14 minutes, doing it for Red Nose Day; it was not an easy write.”
Liam Neeson, Bill Nighy, Colin Firth, Thomas Brodie-Sangster, Olivia Olsen, Rowan Atkinson, Martine McCutcheon and Hugh Grant have all filmed scenes for the spin-off.
A teaser trailer was released last week showing Andrew Lincoln being reunited with the white cue cards he used to profess his love for Keira Knightley's character Juliet.
Juliet was embroiled in a love triangle when she discovered Mark (Lincoln), who was best friends with her husband Peter (Chiwetel Ejiofor), was in love with her.
Knightley was pictued on the new set wearing pyjamas as she cuddled up to Ejiofor on a sofa.
But with Lincoln being pictured filming with a stack of cue cards fans have been left wondering if Mark still harbours feelings for her.
Comic Relief is on BBC One on March 24.