The veteran actors will star as Douglas and Hillary Potts, who arrive in the Yorkshire village in April to visit their pregnant daughter.
Preston's 40-year acting career includes roles in Victoria Wood comedies such as Acorn Antiques and Dinnerladies. He recently played Grandpa Jim in ITV1 drama About Thomas.
Wilcox is best known for her role on 1970s sitcom Man About The House. She recently appeared in Footballers' Wives and Doctors.
Kathleen Beedles, Emmerdale's series producer, said: "We are thrilled to welcome Duncan and Paula to the cast. They bring an enormous amount of acting experience with them and viewers will be gripped by their dramatic arrival."
The pair start filming next week and will be on screen for around three weeks.
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