Mycroft has long been one of the biggest mysteries of the show, and whilst it's tempting to analyse every single Mycroft scene as just Mark Gatiss adding more and more to his acting CV, Mycroft and Sherlock's relationship has significantly stepped up a notch in Series 3. The conversation in 'The Empty Hearse' about how they both thought Sherlock was stupid is actually far more than childhood nostalgia, as it's Mycroft that appears as the voice in Sherlock's mind palace, pushing him to find the Mayfly Man. The realisation that Sherlock has about his brother perhaps being lonely reveals as much about his new-found reliance on friends as it does the difference between him and his brother. And the final image that Mycroft has of his brother, standing as a small child with hands raised above his head suggests something that was first mentioned in 'A Study in Pink'; Mycroft really does care about him. It's all very touching to pull apart the Holmes family - even to include cameos from Cumberbatches Snr - but what's so brilliant about it, is that it shows new sides to Sherlock. It shows his past, it shows how the man works, and it digs at the humanity beneath the icy exterior. In many ways, Mark Gatiss is the only man who could play Mycroft; he's the only actor who knows exactly what the writers want to bring out in Sherlock's character, and Mycroft is the perfect device to do that.
