The reading benefits the Actors' Benevolent Fund. Prince Charles is a patron of the charity, which supports actors, actresses, and stage managers experiencing financial hardship.
"I was terribly nervous," laughs Gatwa in the trailer, best known for his breakout role as Eric in the Netflix show Sex Education.
Prince Charles is shown in the sneak peek laughing as the director asks him to step to his left.
The reading is set for release on Christmas Eve UK time.
People magazine reports Charles came up with the idea himself when asked by the fund for an idea to help the charity over the Christmas season.
The Duchess of Cornwall participated in another special reading in May this year, joining New Zealand filmmaking star Taika Waititi for a live reading of James and The Giant Peach.
She said at the time she was "thrilled" to take part in the reading, which raised money for Partners In Health.
She voiced The Ship's Captain, and even had a line that referenced the Queen.
"It's a secret weapon. Holy cats! Send a message to the Queen at once," she read.
The Duke and Duchess of Cornwall are spending Christmas together at Gloucestershire, and along with the rest of the royal family will not attend the Sandringham Christmas service due to the pandemic.
