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The brief preview, cut from the final draft of his forthcoming instalment, features the Doctor and Amy investigating a mysterious bowl of food.
"The Doctor has just been given a bowl of something to eat," wrote Gaiman on his blog. "Something... possibly... alien."
The writer also revealed that the tenth draft of the script was rewritten by showrunner Steven Moffat.
Gaiman said: "Steven Moffat came to my rescue when I felt like I couldn't even pick it up again, and for that, he is a hero."
He added: "It hasn't really changed that much. It just gets tighter and, I hope, more like itself."
He explained that the episode is now nearing the production stage, with a table read through "about ten days away".
The new series of Doctor Who is expected to air on BBC One in early 2011.
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