Answering fan queries for the Liverpool Daily Post, Fay added that Miracle Day will "still have the same basic tone".
"Russell [T. Davies] and Julie Gardner remain in charge," he explained. "[But] there are more American characters in it, so obviously it'll probably feel more American."
Fay added that writing the ninth episode of the ten-part run had been "daunting".
"The other writers in the room were phenomenal," he said. "I hadn't met any of them before, apart from Russell Davies, but I knew their work. We got on great, I think."
The writer, who previously contributed two episodes to Torchwood: Children of Earth, also criticised fans who were unhappy with the death of Ianto Jones (Gareth David-Lloyd).
"A universe in which fictional characters aren't 'allowed' to die is ridiculous and limiting," he argued. "When death is a reality in a drama, the jeopardy is greater [and] the stakes are greater. In Torchwood, anyone can die. That's a good thing."
Torchwood: Miracle Day will air on BBC One in the UK and Starz in the US.