As the Twelfth Doctor's casting process, and indeed the Sixth's, has shown us, appearing on 'Who' in another role does no harm to your chances of one day pocketing a TARDIS key yourself. Did you spot our erstwhile hero's cameos in ' The Eleventh Hour' & ' The Power Of Three'? Small, but perfectly formed and no doubt a dream come true for him and anyone else who's been asked to appear & accepted across 50 years of making the impossible possible. Plus the BBC would save a fortune on scientific researchers, a fact that most likely wouldn't escape the pencil-pushers or the drama department while there's a recession on & all.