Although the Doctor encountered a few people more than once on his travels in the classic days (Sabalom Glitz, for example, met the Sixth and Seventh Doctors), there were rarely any characters who appeared in more than one story. Ever since NuWho set up Rose's mum Jackie as a recurring character, the series has delighted in bringing characters back for new adventures and new situations. The most recent example is, of course, the love-em-or-hate-em (and you should love 'em!) Paternoster gang. These characters, one suspects, were meant to be a bit of a gag but wound up becoming some of the most popular and enduring characters of the new series to date, so much so that they have now encountered two incarnations of the Doctor (the Eleventh and Twelfth). But beyond the Paternosters, the show has had the young kids who Clara babysat (quickly jettisoned), Rory's brilliant dad, and even James Corden's Craig counts. And who's to say he won't be back? Imagine the shock of realising his best bud has turned into a grumpy old man! In the classic series, there was a sense of "one and done" with guest stars and characters who weren't villains. Now, with a new ethos and a new energy to the show, characters are allowed to simmer for a while away from the stories in which they were introduced. And this is a very good thing indeed.
