I think there was a friendship instantly between Martin and Benedict. They’re not at all like the parts they play, but the relationship between them really informs the much pricklier on-screen relationship. Because, obviously, Sherlock’s a quite cruel man at times and John’s quite a put-upon man at times, and you won’t buy that, you won’t enjoy that, unless you absolutely feel in every scene, in every heartbeat, that there is that proper underlying warmth, that real, proper, solid friendship. That friendship has actually happened between Benedict and Martin. And the value of that you get on-screen.
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Steven Moffat