PeterCapaldi’sfinalepisodeofDoctorWhothisChristmaswillfeaturePearlMackieasBillPottsandbecalledTwiceUponaTime,itwasrevealedthisevening(23July)duringacastpanelatSanDiegoComicCon.ViewerswillhavetowaituntilChristmastodiscoverexactlyhowBill,whowillappearthroughouttheepisode,makesherreturn.