At the recent San Diego Comic Con, Steven Moffat said his favorite Doctor Who episode is "The Christmas Invasion." It's David Tennant's "Castrovalva," the episode that introduced Peter Davison's Fifth Doctor. As in that episode, the Doctor's companions must combat an alien threat while he is recovering from his regeneration, when he has transformed himself and gained a new appearance to combat serious injury. The Fifth and Tenth Doctors share many characteristics, as shown in the 2007 "Time Crash" scene with Davison and Tennant. Of course, Tennant incorporates traits of all the previous Doctors in his performance, especially Tom Baker's Fourth Doctor, since the role is the same person played by different actors as the Time Lord regenerates. But the Tenth Doctor tells the Fifth, "You were my Doctor," something that might be reflected in the exchange between the actors Tennant and Davison. In fact, Doctor Who Magazine's Andrew Pixley gives a good account of the historic meeting of the two Doctors in the new Doctor Who Companion: Series Four, which also has lots of information on the making of the show in 2008.