It's an all-important catchword in TV circles - "chemistry". Get the mix right between presenters, and the audience will welcome them in. Get it wrong, and shows can easily flop. But how do TV executives decide whether a combination will work? Is it pure chance, or are there ways to determine whether sparks will fly for the camera? In this special programme, Steve Hewlett talks to agent Michael Foster, TV executive Lorraine Heggesey, TV critic Kevin O'Sullivan, and famous successful duo Richard and Judy about how to create that very special something between hosts.Producer: Katy Takatsuki Editor: Andrew Smith.