from fans when the Doctor from the fifth series finale "The Big Bang" appeared in the earlier episode "Flesh and Stone". Moffat decided to "rest" the Doctor's arch-enemies the Daleks for the series, as being the "most frequent" enemies of the show made them "the most reliably defeatable enemies in the universe". The series continues to build the mystery of the Silence, which had been introduced in the fifth series. Moffat did not wish to end the arc in the previous series, as he felt it would be "more fun" to continue it. Moffat had planned the revelation about River Song "for a long time"; when creating Amy's character, he chose "Pond" for her last name to create a link. Moffat intended for the "answer to be as complicated as the question". Moffat informed Kingston of the secrets of her character at the end of the previous series and she was not allowed to tell anyone; Smith, Gillan, and Darvill were unaware of the identity of her character. Song's identity was kept in top secrecy; the script read at the read-through of "A Good Man Goes to War" had a false ending, and only a select few were issued the real script. Writing Matthew Graham was not able to write an episode as he was slated to for the fifth series because he did not believe he would have enough time. Moffat then asked him to write the two-part episode "The Rebel Flesh"/"The Almost People". Neil Gaiman had written "The Doctor's Wife" for the previous series, but due to budget constraints it was replaced with "The Lodger". This necessitated changes to the script, including the addition of Rory as a companion. During the production process the order of the third, fourth and ninth episodes was changed. "The Curse of the Black Spot" was swapped with "Night Terrors" because Moffat felt that the second half of the series was too dark. This necessitated minor changes for the episodes to fit into the storyline, most notably in "Night Terrors". Moffat wanted to start the series with a two-parter that had gravity and a wider scope in plot. He also aimed to make them some of the darkest episodes. In contrast, the following episode, "The Curse of the Black Spot", would allow the characters to "kick back and have some fun". Gaiman's episode was based around the TARDIS, and allowing the Doctor and