Last month, the BBC announced that "Doctor Who" star Matt Smith had decided to leave the 50-year-old British science fiction show after this year's special Christmas episode. So how goes the search for the next Doctor? "Well, it's always just terrifying," says "Doctor Who" executive producer Steven Moffat, who recruited Smith to replace his predecessor, David Tennant, back in late 2008. "If you're a 'Doctor Who' fan, as I have been all my life, you've been doing fantasy casting for this part for as long as you can remember. But when you're suddenly faced with the reality that you are going to sit there and you are going to make that decision it does feel absolutely chilling. There's a very big range of people who could play it and different ways you could go with it. We must get this right. One false move and the show's over."