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Mark Gatiss is probably used to it (coz it's what he does) but I would find it very weird writing lines that I know I will later have to deliver. Of course I'm not an actor so my assumption could be totally wrong. It's one thing to write & direct (which apparently happens a lot); it's another to write & then act it out. I can just imagine the feedback he might get from his director: "Hey Mark, look I know you wrote it but your delivery was completely off." (Mark: really?)
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But the writer is not really responsible for the acting. I'm no expert either but I was under the impression that actors and directors experiment a lot with the delivery in the rehearsals. So it's really a collaborative process.
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I was thinking that maybe Mark doesn't so much think about how *he* will sound delivering the lines as he writes them, but they're just there in Mycroft's voice. I'm sure everyone's brain is wired differently, but sometimes when I read something, I don't so much "hear" the actor's voice in my head but more my own voice imagining to be the character. I'm also quite sure that, as an actor, you know how to separate yourself from the character you play.
But since I've never written anything that I would act out later myself, this is all just theory.