Apparently Matt Smith also played one of Billie Piper's tricks in her other series. Wonder if they cast him because he's got lots of ex-girlfriends/flatmates who can be cast as guest stars, like that scruffy littleScotsman? Actually, more seriously, wonder if the youth thing will apply to future guest stars? Emma Watson, Anna Popplewell? Rupert Grint? The other one from Harry Potter, whose name I can never remember... If you and not_so are companions, who would be the Doctor? Can't be me, I'm the Brig. Packer? Bauble? Grandelf, obviously, would have to be Adric.