Clive Merrison prepares to approach the microphone after opening a sound-effects sash window and vocally-acting climbing through it. BBC studio manager Tim Sturgeon is about to provide the effect of Holmes's feet hitting the gravel path. A slight gasp from Merrison as the impact occurs, and the illusion is perfect.

There is always at least one "spot fx" operator on the studio floor with the cast, providing sounds which their hand-held scripts prevent them from making for themselves. It's a skilled job, calling for inventiveness, timing and the ability to weave silently in and out of the action without knocking over the actors.