Looks like my Doctor Who campaign with Jack and Shelly (and Tom?) is a go for Saturday the 25th. I'm planning to have them make up their own characters rather than play the Doctor and companions. As much fun as it is to run the familiar stars in one-off sessions, I think the focus on imitation would stifle any kind of character development in a campaign. Besides, I'm always eager to meet the new faces that my friends put on at the game table, and they wouldn't be new if they all came from the show.
Already got some ideas for hurling them into the Time Vortex, but I want to see their characters before I set it in stone. I'm putting together a some details of a setting and some creatures & NPC's for a first adventure, which hopefully I'll be able to arrange on the spot for our first session. Jack & Shelly don't have the books, though, so we'll also be making characters at the table, and I suppose it's possible that they'll come up with something that'll require more detailed adaptation. We'll see. One of the things I love about this game is that it's really easy to improvise with.