Casting CelebritiesWhen you're a known actor who is hired for "Doctor Who," whether as a Doctor or companion, it can cause some fans to be excited or nervous. Jessica and Phillip examine those actors who came to the franchise with notable previous roles and how that affected their time in the TARDIS. From one-hit wonder Billie Piper to comedianne Catherine Tate, they co-hosts examine how it all worked out.In their look into the TARDIS library, the couple examines Peter Davison's first time in the cricket outfit in "Castrovalva." Davison was coming off a memorable role on "All Creatures Great and Small..." to become to Fifth Doctor, how did his first serial cement him as the Doctor?Which "Doctor Who" casting surprised you the most?