Donna Noble is played through improv.
 After a conversation between Catherine Tate and the BBC's Jane Tranter, in which Tate expressed an interest in returning, Davies rewrote season four to bring Donna back as the new full-time companion instead
== Casting ==As indicated by David Tennant in his series two video diary (included in the DVD box set), the casting of Catherine Tate was kept secret; her scene in "Doomsday" was filmed with minimal crew

