Sherlock Holmes is in his ­living room, playing Bach. I study his movements, his bow-arm and his fingers. I am working as Benedict Cumberbatch's violin coach and they're filming scenes for series two of Sherlock. When I give a tip, ­Cumberbatch consumes it entirely. Information is his quarry; no wonder he's such a good Sherlock.

I was sitting outside in the sun at AIR Lyndhurst studios in London when we first met: he strode up, motorbike gear on, helmet in hand. I lent him my spare violin and as he attached his leather belt to it and slung it over his shoulder, I realised that violins look much cooler when worn by bikers.

A lot of the first lesson is spent on stance and hand positions. He's quick, focused. By our ­second lesson, ­Cumberbatch makes a surprisingly good sound.

By the third I am becoming familiar with his "locked on to target" eyes, and when it's best to stand back and leave him to work it out for himself. He handles the violin carefully: "How would you lift it to your chin?", "How would you play around with the bow?", "Which way would you put it down on a chair?". Everyone has gone home, except a cleaner, who comes in, bucket in hand and backs out apologising. Cumberbatch apologises to her for using the room for so long; he does so with such grace I am glad to be coaching him.

Eos Chater of Bond. Photograph: Han Myung-Gu

In the scheme of things the violin isn't a big deal in Sherlock – but still I feel the responsibility of "violin department". Often, between scenes in filming, his only moments of rest, he asks for lessons. One time I'm teaching him on the side of the set, in the dark, dodging crew hefting lights around, with him dressed in a bed sheet. This is fine, I tell myself. Totally fine.

On set I need to see him; to play when he lifts his violin and stop when he stops. And he needs to see me; to copy my bowings, to ghost what I'm doing. In one scene I have to stand outside on two boxes on a scissor lift, watching him while he watches me out of the window.

Finally, having avoided reading the script, I sat down to watch the first episode on Sunday: it felt great to have been a small part of it.

• To read a full, unabridged version of this article, go to Eos Chater's website: earthobservingsystem.tumblr.com