Gaffney, who is best known for his role as EastEnders' Robbie Jackson, will play Gavin Josef, a cocky wide-boy who is connected to an assault investigation. Rumours suggesting that the actor is to reprise his EastEnd role were quashed earlier this month.
Meanwhile, Birds of a Feather star Linda Robson - Tracey Stubbs in the BBC comedy - is to appear in Sun Hill alongside Bad Girls' Nicola Stapleton - Janine Nebeski in the show - as characters Christine Haworth and Julie Barnes, respectively, to form a drug-addicted mother-daughter duo.
Linda's former on-screen partner Pauline Quirke also made a cameo appearance in The Bill last year as the mysterious Lesley Palmer and is to play a homeless person in Casualty in mid-October.
All three artists have filmed their parts and are scheduled to be on-screen in November.