Instead, Lee Mack (The Miser) will take over Fry's role, joining a cast including James Norton, Mina Anwar, Natalie Casey, Sasha Frost, Marie Lawrence, James McNicholas and Lizzie Muncey will be Mathew Horne, Emily Atack, Adelayo Adedayo, Lucy Briggs-Owen and Jake Ferretti.
The charity semi-staged shows will be raising funds for Acting For Others, the Equity Charitable Trust and the Theatre Development Trust, run by the Society of London Theatre and UK Theatre.
The show has sound, music and effects by Alexandra Faye Braithwaite, Annie May Fletcher and Sophie Galpin, with direction from Giles Croft. Animation is by Dean Kendrick and Mark Kendrick for Dusthouse, while the piece is produced by Huddersfield's Lawrence Batley Theatre.
The Understudy runs on 7 and 8 December at 7.30pm at the Palace Theatre, with tickets on sale now.
You can watch Mack and Fry having a chat about taking over the role below: