Although 
Mark Gatiss has already appeared twice in 
Doctor Who (Professor Lazarus in 
The Lazarus Experiment and an uncredited voice in 
Victory of the Daleks), a recent 
interview  with Simon Fisher-Becker (Dorium Maldavar in 
The Pandorica Opens and 
A Good Man Goes To War) suggested that the writer would be making another appearance in the show (specifically 
Episode 13).
As it's unlikely Gatiss will be appearing as 'normal', Blogtor wonders if the pictures included here, from both trailers from 
Series 6, Part 2 are of the 
Sherlock writer in action. Judge for yourself. Click on the pics for bigger versions. Mark Gatiss has also written a story for the series, 
Night TerrorsETA: The 
Radio Times has confirmed that Mark Gatiss will star in in the episode now titled, 
The Wedding of River Song.
ETA (Sep 21): Blogtor can confirm that is IS Gatiss. His character name is Gantok.