Heartstopper creator Alice Oseman made the difficult decision to recast Sarah Nelson rather than exclude the character from the film, stating that Olivia Colman was not able to join for the film. Oseman explained that Sarah Nelson's presence was considered essential to the Heartstopper Forever film story, noting that when beginning to work on the screenplay, she knew how important it was that Sarah appeared. The exact reason why Colman was unable to commit to the film has not been disclosed.
In season three of Heartstopper, the story was tweaked to avoid Sarah Nelson's appearance, with Hayley Atwell joining as Nick's aunt Diane. However, Oseman emphasized that for the film, it felt nonsensical for her to be absent from this final chapter, given some of the emotional struggles Nick faces. The specific nature of these emotional struggles and the broader plot details of Heartstopper Forever remain unknown.
Multiple cast members are confirmed to reprise their roles in Heartstopper Forever, including Joe Locke as Charlie Spring, Kit Connor as Nick Nelson, Yasmin Finney as Elle Argent, Will Gao as Tao Xu, Corinna Brown as Tara Jones, Kizzy Edgell as Darcy Olsson, Tobie Donovan as Isaac Henderson, Rhea Norwood as Imogen Heaney, Jenny Walser as Tori Spring, Fisayo Akinade as Mr. Ajayi, Nima Taleghani as Mr. Farouk, and Leila Khan as Sahar.
Additionally, Sir Derek Jacobi will feature in Heartstopper Forever in a cameo role, though the nature of his appearance has not been revealed. The release date for the film is also currently unknown.