Becca's (Simone James) secret has so far remained undiscovered but Turner told What's On TV that that was about to change, as Becca herself reveals all to Stacey's mother Jean (Gillian Wright).
"It's actually quite chilling, Jean's scared," she said. "Becca tells Jean that Stacey is hers now."
Turner revealed that viewers would see Becca convince Stacey to wash away her medication.
"She encourages Stacey to believe that without the medication she'll be able to feel proper emotions again and will appreciate the joys of being with Lily more," she said. "Becca tells her the pills are the doctors' way of controlling her."
She also said that Stacey does not "for a moment" consider that Becca is lying.
"Becca's messing with Stacey's life more and more but Stacey can't see it," she added.
Of how Stacey would react if she discovered Becca's betrayal, Turner teased: "There will be no forgiveness."
It was announced in April that both Turner and Wright are to exit the BBC soap.