McCallum has been promoted to head of drama. Leatherland will now act as development head.
Leatherland previously worked as a script executive at the BBC on Eastenders and Sky Drama where he worked on Riviera, Britannia, Tin Star, Guerrila, Jamestown and The Last Dragonslayer.
Aude Konan has been appointed as development assistant.
Mick Pilsworth, chief executive at Chrysalis Vision, said: “Our expanded drama team comprises the essential qualities of experience, creativity and talent; and we look forward to creating high quality, successful television drama with them.”