Talking to OK, Wheeler - who plays Nicola King - said that it was a shock to find out which of the soap's main characters would bow out permanently during the 40th anniversary celebrations.
"We were devastated when we found out who it was," she added.
Wheeler also admitted that she is nervous about the implications of the episode being broadcast live.
"Sometimes if I forget a line, I swear," she said. "I'm terrible for that. Obviously I can't do that on live TV. That would be really bad."
Earlier today, a new teaser video was released showing the soap's stars discussing next week's live episode.
Emmerdale's anniversary celebrations air all next week on ITV1. Emmerdale Live airs on Wednesday, October 17 at 7pm.
> 'Emmerdale' backstage webcams for live 40th anniversary episode
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> 'Emmerdale' 40th anniversary: The live episode in numbers