River Song (Doctor Who)
The character was first created to help the plot of "Silence in the Library" to make more sense. Moffat knew that the team of archaeologists would have to trust the Doctor, but the Doctor could not get them to understand how he had gotten into the closed library. Because of this, Moffat wanted for the Doctor to know one of the archaeologists but he thought that the idea was "dull". He chose to have one of them know him. When Kingston returned to the show, Moffat told her some details about her character's back-story. This was information that Smith, Gillan and Darvill did not learn until later. Kingston knows her character cannot be killed off because her death was seen in her first episode. Each episode she is in after that is earlier in her personal history.